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.

TODO.md

@@ -28,6 +28,11 @@ databases currently grow without bound.
 
 # Completed Steps
 
+- 2026-08-09 Archive writer lifecycle (#89): deleting a webhook (or its
+  last `database` target) evicts the cached archive writer and closes
+  its handle while deliberately leaving `archive-{webhookID}.db` on
+  disk, and a new `ArchiveSweeper` prunes idle archives on the existing
+  `RETENTION_SWEEP_INTERVAL` without ever creating an archive file
 - 2026-08-07 Update golangci-lint to v2.12.2 (Docker image digest in
   `Dockerfile`, release-archive sha256 pins in `script/bootstrap`),
   adopt the canonical `.golangci.yml` (v2 `linters.settings` layout so