Chore/expire files - #19

[1.10.0] — unreleased

A backward-compatible minor release adding file expiry / temp files.
Additive; no API removals.

✨ Added

• Expiring / temp files (SetFileTTL, SetFileExpiry, PurgeExpired,
  WithExpirySweeper, and upload-with-TTL: UploadFileWithTTL /
  UploadFileWithExpiry / UploadFileFromReaderWithTTLContext). Give a file a
  TTL and buckt permanently deletes it —
  blob, image derivatives, and metadata row — once it's due, emitting a
  file.purged event. A new indexed expires_at column (schema migration v6,
  additive) makes the sweep a cheap ranged query. The sweep is app-driven by
  default (call PurgeExpired from your own scheduler); WithExpirySweeper
  opts into a built-in background ticker (stopped by Client.Close). Built to
  extend toward general scheduled actions (e.g. "email this after 24h") on the
  same event-backed sweep. The web client exposes it too: PUT /expiry/:file_id
  (ttl or absolute RFC3339) and POST /purge-expired on the API, and in the UI
  a per-file "Expire in 1h/24h/7d" menu with an expiry badge (POST /web/set-ttl/:file_id, POST /web/purge-expired). The ui example gains an
  -expiry=<interval> flag (make ui EXPIRY=30s) to run the sweeper, and a new
  example/expiry/headless walks the TTL → purge lifecycle.