Evict archive writers on deletion and sweep idle archives (closes #89) (#95)

Per-webhook archive writers are now evicted when the webhook or its last
database target is deleted, and a background sweeper prunes expired rows from
idle archives that no longer receive writes. Archive files themselves are never
deleted.

TODO.md

@@ -10,12 +10,13 @@
 
 # Status
 
-pre-1.0. No git tags exist. main (afe88c6) is a working webhook proxy
+pre-1.0. No git tags exist. main (4f5ecb1) is a working webhook proxy
 with auth, CSRF/SSRF protections, login rate limiting, Slack target,
-policy compliance (#6), and pinned lint tooling (#55). Note: TODO.md was
-deliberately deleted from this repo in f9a9569 (2026-03-01, #6); its
-content was folded into the README TODO section, which this draft
-reconstructs as of 2026-07-06.
+event retention (#63), the database archiving target (#43), the admin
+password change flow (#65), policy compliance (#6), and pinned lint
+tooling (#55). Note: TODO.md was deliberately deleted from this repo in
+f9a9569 (2026-03-01, #6); its content was folded into the README TODO
+section, which this draft reconstructs as of 2026-07-06.
 
 # Next Step
 
@@ -34,6 +35,11 @@ databases currently grow without bound.
   killed both roughly fifteen seconds after boot: the proxy silently
   stopped delivering webhooks entirely, and the reaper never ran a
   single sweep under its default one-hour interval
+- 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