feat: implement per-webhook event databases

Split data storage into main application DB (config only) and
per-webhook event databases (one SQLite file per webhook).

Architecture changes:
- New WebhookDBManager component manages per-webhook DB lifecycle
  (create, open, cache, delete) with lazy connection pooling via sync.Map
- Main DB (DBURL) stores only config: Users, Webhooks, Entrypoints,
  Targets, APIKeys
- Per-webhook DBs (DATA_DIR) store Events, Deliveries, DeliveryResults
  in files named events-{webhook_uuid}.db
- New DATA_DIR env var (default: ./data dev, /data/events prod)

Behavioral changes:
- Webhook creation creates per-webhook DB file
- Webhook deletion hard-deletes per-webhook DB file (config soft-deleted)
- Event ingestion writes to per-webhook DB, not main DB
- Delivery engine polls all per-webhook DBs for pending deliveries
- Database target type marks delivery as immediately successful (events
  are already in the dedicated per-webhook DB)
- Event log UI reads from per-webhook DBs with targets from main DB
- Existing webhooks without DB files get them created lazily

Removed:
- ArchivedEvent model (was a half-measure, replaced by per-webhook DBs)
- Event/Delivery/DeliveryResult removed from main DB migrations

Added:
- Comprehensive tests for WebhookDBManager (create, delete, lazy
  creation, delivery workflow, multiple webhooks, close all)
- Dockerfile creates /data/events directory

README updates:
- Per-webhook event databases documented as implemented (was Phase 2)
- DATA_DIR added to configuration table
- Docker instructions updated with data volume mount
- Data model diagram updated
- TODO updated (database separation moved to completed)

Closes #15

internal/database/webhook_db_manager_test.go

@@ -0,0 +1,294 @@
+package database
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/uuid"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/globals"
+	"sneak.berlin/go/webhooker/internal/logger"
+	pkgconfig "sneak.berlin/go/webhooker/pkg/config"
+)
+
+func setupTestWebhookDBManager(t *testing.T) (*WebhookDBManager, *fxtest.Lifecycle) {
+	t.Helper()
+
+	fs := afero.NewMemMapFs()
+	testConfigYAML := `
+environments:
+  dev:
+    config:
+      port: 8080
+      debug: false
+      dburl: "file::memory:?cache=shared"
+    secrets:
+      sessionKey: d2ViaG9va2VyLWRldi1zZXNzaW9uLWtleS1pbnNlY3VyZSE=
+configDefaults:
+  port: 8080
+`
+	require.NoError(t, afero.WriteFile(fs, "config.yaml", []byte(testConfigYAML), 0644))
+	pkgconfig.SetFs(fs)
+
+	lc := fxtest.NewLifecycle(t)
+
+	globals.Appname = "webhooker-test"
+	globals.Version = "test"
+	globals.Buildarch = "test"
+
+	g, err := globals.New(lc)
+	require.NoError(t, err)
+
+	l, err := logger.New(lc, logger.LoggerParams{Globals: g})
+	require.NoError(t, err)
+
+	dataDir := filepath.Join(t.TempDir(), "events")
+
+	cfg := &config.Config{
+		DBURL:      "file::memory:?cache=shared",
+		DataDir:    dataDir,
+		SessionKey: "d2ViaG9va2VyLWRldi1zZXNzaW9uLWtleS1pbnNlY3VyZSE=",
+	}
+	_ = cfg
+
+	mgr, err := NewWebhookDBManager(lc, WebhookDBManagerParams{
+		Config: cfg,
+		Logger: l,
+	})
+	require.NoError(t, err)
+
+	return mgr, lc
+}
+
+func TestWebhookDBManager_CreateAndGetDB(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhookID := uuid.New().String()
+
+	// DB should not exist yet
+	assert.False(t, mgr.DBExists(webhookID))
+
+	// Create the DB
+	err := mgr.CreateDB(webhookID)
+	require.NoError(t, err)
+
+	// DB file should now exist
+	assert.True(t, mgr.DBExists(webhookID))
+
+	// Get the DB again (should use cached connection)
+	db, err := mgr.GetDB(webhookID)
+	require.NoError(t, err)
+	require.NotNil(t, db)
+
+	// Verify we can write an event
+	event := &Event{
+		WebhookID:    webhookID,
+		EntrypointID: uuid.New().String(),
+		Method:       "POST",
+		Headers:      `{"Content-Type":["application/json"]}`,
+		Body:         `{"test": true}`,
+		ContentType:  "application/json",
+	}
+	require.NoError(t, db.Create(event).Error)
+	assert.NotEmpty(t, event.ID)
+
+	// Verify we can read it back
+	var readEvent Event
+	require.NoError(t, db.First(&readEvent, "id = ?", event.ID).Error)
+	assert.Equal(t, webhookID, readEvent.WebhookID)
+	assert.Equal(t, "POST", readEvent.Method)
+	assert.Equal(t, `{"test": true}`, readEvent.Body)
+}
+
+func TestWebhookDBManager_DeleteDB(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhookID := uuid.New().String()
+
+	// Create the DB and write some data
+	require.NoError(t, mgr.CreateDB(webhookID))
+	db, err := mgr.GetDB(webhookID)
+	require.NoError(t, err)
+
+	event := &Event{
+		WebhookID:    webhookID,
+		EntrypointID: uuid.New().String(),
+		Method:       "POST",
+		Body:         `{"test": true}`,
+		ContentType:  "application/json",
+	}
+	require.NoError(t, db.Create(event).Error)
+
+	// Delete the DB
+	require.NoError(t, mgr.DeleteDB(webhookID))
+
+	// File should no longer exist
+	assert.False(t, mgr.DBExists(webhookID))
+
+	// Verify the file is actually gone from disk
+	dbPath := mgr.dbPath(webhookID)
+	_, err = os.Stat(dbPath)
+	assert.True(t, os.IsNotExist(err))
+}
+
+func TestWebhookDBManager_LazyCreation(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhookID := uuid.New().String()
+
+	// GetDB should lazily create the database
+	db, err := mgr.GetDB(webhookID)
+	require.NoError(t, err)
+	require.NotNil(t, db)
+
+	// File should now exist
+	assert.True(t, mgr.DBExists(webhookID))
+}
+
+func TestWebhookDBManager_DeliveryWorkflow(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhookID := uuid.New().String()
+	targetID := uuid.New().String()
+
+	db, err := mgr.GetDB(webhookID)
+	require.NoError(t, err)
+
+	// Create an event
+	event := &Event{
+		WebhookID:    webhookID,
+		EntrypointID: uuid.New().String(),
+		Method:       "POST",
+		Headers:      `{"Content-Type":["application/json"]}`,
+		Body:         `{"payload": "test"}`,
+		ContentType:  "application/json",
+	}
+	require.NoError(t, db.Create(event).Error)
+
+	// Create a delivery
+	delivery := &Delivery{
+		EventID:  event.ID,
+		TargetID: targetID,
+		Status:   DeliveryStatusPending,
+	}
+	require.NoError(t, db.Create(delivery).Error)
+
+	// Query pending deliveries
+	var pending []Delivery
+	require.NoError(t, db.Where("status = ?", DeliveryStatusPending).
+		Preload("Event").
+		Find(&pending).Error)
+	require.Len(t, pending, 1)
+	assert.Equal(t, event.ID, pending[0].EventID)
+	assert.Equal(t, "POST", pending[0].Event.Method)
+
+	// Create a delivery result
+	result := &DeliveryResult{
+		DeliveryID: delivery.ID,
+		AttemptNum: 1,
+		Success:    true,
+		StatusCode: 200,
+		Duration:   42,
+	}
+	require.NoError(t, db.Create(result).Error)
+
+	// Update delivery status
+	require.NoError(t, db.Model(delivery).Update("status", DeliveryStatusDelivered).Error)
+
+	// Verify no more pending deliveries
+	var stillPending []Delivery
+	require.NoError(t, db.Where("status = ?", DeliveryStatusPending).Find(&stillPending).Error)
+	assert.Empty(t, stillPending)
+}
+
+func TestWebhookDBManager_MultipleWebhooks(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+	defer func() { require.NoError(t, lc.Stop(ctx)) }()
+
+	webhook1 := uuid.New().String()
+	webhook2 := uuid.New().String()
+
+	// Create DBs for two webhooks
+	require.NoError(t, mgr.CreateDB(webhook1))
+	require.NoError(t, mgr.CreateDB(webhook2))
+
+	db1, err := mgr.GetDB(webhook1)
+	require.NoError(t, err)
+	db2, err := mgr.GetDB(webhook2)
+	require.NoError(t, err)
+
+	// Write events to each webhook's DB
+	event1 := &Event{
+		WebhookID:    webhook1,
+		EntrypointID: uuid.New().String(),
+		Method:       "POST",
+		Body:         `{"webhook": 1}`,
+		ContentType:  "application/json",
+	}
+	event2 := &Event{
+		WebhookID:    webhook2,
+		EntrypointID: uuid.New().String(),
+		Method:       "PUT",
+		Body:         `{"webhook": 2}`,
+		ContentType:  "application/json",
+	}
+	require.NoError(t, db1.Create(event1).Error)
+	require.NoError(t, db2.Create(event2).Error)
+
+	// Verify isolation: each DB only has its own events
+	var count1 int64
+	db1.Model(&Event{}).Count(&count1)
+	assert.Equal(t, int64(1), count1)
+
+	var count2 int64
+	db2.Model(&Event{}).Count(&count2)
+	assert.Equal(t, int64(1), count2)
+
+	// Delete webhook1's DB, webhook2 should be unaffected
+	require.NoError(t, mgr.DeleteDB(webhook1))
+	assert.False(t, mgr.DBExists(webhook1))
+	assert.True(t, mgr.DBExists(webhook2))
+
+	// webhook2's data should still be accessible
+	var events []Event
+	require.NoError(t, db2.Find(&events).Error)
+	assert.Len(t, events, 1)
+	assert.Equal(t, "PUT", events[0].Method)
+}
+
+func TestWebhookDBManager_CloseAll(t *testing.T) {
+	mgr, lc := setupTestWebhookDBManager(t)
+	ctx := context.Background()
+	require.NoError(t, lc.Start(ctx))
+
+	// Create a few DBs
+	for i := 0; i < 3; i++ {
+		require.NoError(t, mgr.CreateDB(uuid.New().String()))
+	}
+
+	// CloseAll should close all connections without error
+	require.NoError(t, mgr.CloseAll())
+
+	// Stop lifecycle (CloseAll already called, but shouldn't panic)
+	require.NoError(t, lc.Stop(ctx))
+}