Evict archive writers on deletion and sweep idle archives (closes #89)

The per-webhook archiveWriter registry in the database delivery target
was never evicted, so a deleted webhook's writer -- and any archive
file handle open within its debounce window -- lingered for the process
lifetime. Separately, expiry pruning ran only when an archive was
(re)opened, and reopens only happen on writes, so an archive belonging
to a webhook that stopped receiving events kept its expired rows
forever.

Eviction: a new one-method delivery.WebhookEvictor interface (kept
separate from Notifier: archiving lifecycle is not notification) is
implemented by the Engine and injected into the handlers. Deleting a
webhook, or deleting its last database target, drops the writer from
the registry and closes its handle under the writer's own mutex, so
eviction can never race an in-flight write. An evicted writer refuses
further writes rather than reopening a file nothing holds. The archive
file is deliberately left on disk: it is long-term storage an operator
may want to keep or move away, and destroying it as a side effect of
deleting a webhook would be unrecoverable.

Idle sweep: a new ArchiveSweeper, modelled on the event RetentionReaper
(fx lifecycle hooks, cancellable context, WaitGroup, ticker loop),
prunes archives whose database target declares a positive expiry. It
reuses the existing RETENTION_SWEEP_INTERVAL rather than adding a
config key. It never creates an archive -- a missing file is skipped,
and the reopen uses SQLite mode=rw so the file cannot be conjured even
if it disappears mid-sweep -- routes the prune through the per-webhook
writer so its mutex orders the sweep against concurrent writes, and
leaves the archive closed so the move-the-file-away workflow keeps
working. A failure for one webhook is logged and the sweep continues.
Archives with no expiry or the expiry "never" are untouched.

internal/delivery/export_test.go

@@ -328,6 +328,90 @@ func (e *ExportArchiveWriter) DB() *gorm.DB {
 	return e.w.db
 }
 
+// ExportHasArchiveWriter reports whether the database target
+// currently caches an archive writer for a webhook.
+func (e *Engine) ExportHasArchiveWriter(
+	webhookID string,
+) bool {
+	e.dbTarget.mu.Lock()
+	defer e.dbTarget.mu.Unlock()
+
+	_, ok := e.dbTarget.writers[webhookID]
+
+	return ok
+}
+
+// ExportArchiveHandleOpen reports whether the cached archive
+// writer for a webhook holds an open database handle. It
+// returns false when no writer is cached.
+func (e *Engine) ExportArchiveHandleOpen(
+	webhookID string,
+) bool {
+	e.dbTarget.mu.Lock()
+	w, ok := e.dbTarget.writers[webhookID]
+	e.dbTarget.mu.Unlock()
+
+	if !ok {
+		return false
+	}
+
+	w.mu.Lock()
+	defer w.mu.Unlock()
+
+	return w.db != nil
+}
+
+// ExportEnsureArchiveWriter creates (if needed) and returns the
+// archive file path of the cached writer for a webhook, so a
+// test can prime the registry the way a delivery would.
+func (e *Engine) ExportEnsureArchiveWriter(
+	webhookID string,
+) (string, error) {
+	w, err := e.dbTarget.writerFor(webhookID)
+	if err != nil {
+		return "", err
+	}
+
+	return w.path, nil
+}
+
+// NewTestArchiveSweeper builds an ArchiveSweeper backed by the
+// given main database and engine, without the fx lifecycle.
+// Intended for tests.
+func NewTestArchiveSweeper(
+	db *database.Database,
+	eng *Engine,
+	log *slog.Logger,
+) *ArchiveSweeper {
+	return &ArchiveSweeper{
+		db:       db,
+		eng:      eng,
+		log:      log,
+		interval: time.Hour,
+	}
+}
+
+// ExportSweep runs a single archive sweep synchronously for
+// tests.
+func (s *ArchiveSweeper) ExportSweep(ctx context.Context) {
+	s.sweep(ctx)
+}
+
+// ExportStart starts the sweeper's background loop for tests.
+func (s *ArchiveSweeper) ExportStart(ctx context.Context) {
+	s.start(ctx)
+}
+
+// ExportStop stops the sweeper's background loop for tests.
+func (s *ArchiveSweeper) ExportStop() {
+	s.stop()
+}
+
+// ExportSetInterval overrides the sweep interval for tests.
+func (s *ArchiveSweeper) ExportSetInterval(d time.Duration) {
+	s.interval = d
+}
+
 // ExportParseArchiveExpiry exposes parseArchiveExpiry.
 func ExportParseArchiveExpiry(
 	configJSON string,