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/database/export_test.go

@@ -0,0 +1,31 @@
+package database
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"time"
+)
+
+// NewTestRetentionReaper builds a RetentionReaper backed by the given
+// main database and per-webhook database manager, without the fx
+// lifecycle. Intended for tests.
+func NewTestRetentionReaper(
+	db *Database,
+	mgr *WebhookDBManager,
+) *RetentionReaper {
+	return &RetentionReaper{
+		db:        db,
+		dbManager: mgr,
+		log: slog.New(slog.NewTextHandler(
+			os.Stderr,
+			&slog.HandlerOptions{Level: slog.LevelDebug},
+		)),
+		interval: time.Hour,
+	}
+}
+
+// ExportSweep runs a single retention sweep synchronously for tests.
+func (r *RetentionReaper) ExportSweep(ctx context.Context) {
+	r.sweep(ctx)
+}