Add per-webhook event retention reaper (closes #63) (#78)

Enforces each webhook's `RetentionDays` so per-webhook SQLite files no longer grow without bound.

## Reaper

New `RetentionReaper` in `internal/database/retention.go`. A background ticker runs each sweep: it lists all webhooks from the main DB and, for each webhook with a positive `RetentionDays`, opens its per-webhook DB via `WebhookDBManager.GetDB` and deletes every `Event` (and its dependent `Delivery` and `DeliveryResult` rows) whose `CreatedAt` is older than `RetentionDays` days.

- Deletions run in foreign-key-safe order: delivery results, then deliveries, then events.
- Deletes are unscoped (hard deletes) so rows are physically removed and disk is reclaimed, rather than GORM soft-deleting them.
- `RetentionDays <= 0` means retain forever; those webhooks are skipped.
- Webhooks whose per-webhook DB does not yet exist are skipped.

## Config

`internal/config/config.go` gains `RetentionSweepInterval` (env `RETENTION_SWEEP_INTERVAL`, parsed as a Go duration, default `1h`) via a new `envDuration` helper, following the existing env-helper conventions.

## Wiring

`cmd/webhooker/main.go` registers `database.NewRetentionReaper` as an fx provider and forces its construction in `fx.Invoke`. The reaper starts its sweep loop on an fx `OnStart` hook and stops cleanly on `OnStop` via context cancellation, matching the existing lifecycle components.

## Test

`internal/database/retention_test.go` seeds an old event chain (event + delivery + result, 40 days old) and a recent one (1 day old) in a real per-webhook DB and asserts a single sweep removes only the expired chain while keeping the recent one. A second test forces a non-positive `RetentionDays` and asserts an ancient event is retained.

Note: the `Webhook.RetentionDays` column carries `gorm:"default:30"`, so a `0` passed to a GORM `Create` is replaced by the default; the test forces the value with an explicit column update to exercise the retain-forever path. No model changes were made.

Validated with `docker build .` (fmt-check, lint, test, build) exit 0.

Closes #63

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #78
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config_test
 import (
 	"os"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -120,6 +121,100 @@ func testEnvironmentConfigSuccess(
 	assert.Equal(t, isProd, cfg.IsProd())
 }
 
+func TestRetentionSweepInterval(t *testing.T) {
+	tests := []struct {
+		name        string
+		set         bool
+		value       string
+		expectError bool
+		expected    time.Duration
+	}{
+		{
+			name:     "unset uses default",
+			set:      false,
+			expected: time.Hour,
+		},
+		{
+			name:     "valid value is parsed",
+			set:      true,
+			value:    "15m",
+			expected: 15 * time.Minute,
+		},
+		{
+			name:        "unparseable value fails startup",
+			set:         true,
+			value:       "not-a-duration",
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			t.Setenv("WEBHOOKER_ENVIRONMENT", "dev")
+
+			if tt.set {
+				t.Setenv("RETENTION_SWEEP_INTERVAL", tt.value)
+			} else {
+				require.NoError(t, os.Unsetenv(
+					"RETENTION_SWEEP_INTERVAL",
+				))
+			}
+
+			if tt.expectError {
+				testRetentionSweepIntervalError(t)
+			} else {
+				testRetentionSweepIntervalSuccess(t, tt.expected)
+			}
+		})
+	}
+}
+
+func testRetentionSweepIntervalError(t *testing.T) {
+	t.Helper()
+
+	var cfg *config.Config
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+		),
+		fx.Populate(&cfg),
+	)
+
+	assert.Error(t, app.Err())
+}
+
+func testRetentionSweepIntervalSuccess(
+	t *testing.T,
+	expected time.Duration,
+) {
+	t.Helper()
+
+	var cfg *config.Config
+
+	app := fxtest.New(
+		t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+		),
+		fx.Populate(&cfg),
+	)
+	require.NoError(t, app.Err())
+
+	app.RequireStart()
+
+	defer app.RequireStop()
+
+	assert.Equal(t, expected, cfg.RetentionSweepInterval)
+}
+
 func TestDefaultDataDir(t *testing.T) {
 	for _, env := range []string{"", "dev", "prod"} {
 		name := env