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/handlers/handlers_test.go

@@ -30,6 +30,7 @@ func TestHandleIndex(t *testing.T) {
 				return &config.Config{
 					// This is a base64 encoded 32-byte key: "test-session-key-32-bytes-long!!"
 					SessionKey: "dGVzdC1zZXNzaW9uLWtleS0zMi1ieXRlcy1sb25nISE=",
+					DataDir:    t.TempDir(),
 				}
 			},
 			func() *database.Database {
@@ -37,6 +38,7 @@ func TestHandleIndex(t *testing.T) {
 				db := &database.Database{}
 				return db
 			},
+			database.NewWebhookDBManager,
 			healthcheck.New,
 			session.New,
 			New,
@@ -64,12 +66,14 @@ func TestRenderTemplate(t *testing.T) {
 				return &config.Config{
 					// This is a base64 encoded 32-byte key: "test-session-key-32-bytes-long!!"
 					SessionKey: "dGVzdC1zZXNzaW9uLWtleS0zMi1ieXRlcy1sb25nISE=",
+					DataDir:    t.TempDir(),
 				}
 			},
 			func() *database.Database {
 				// Mock database
 				return &database.Database{}
 			},
+			database.NewWebhookDBManager,
 			healthcheck.New,
 			session.New,
 			New,