refactor: split monolithic handlers + testing into domain-grouped modules

handlers/api.rs (1706 LOC) + html.rs (1735 LOC) → 12 domain files:
auth, diary, movies, users, search, watchlist, goals, social,
integrations, helpers + existing import/webhook/wrapup/images/rss.

domain/testing.rs (1309 LOC) → testing/ module:
in_memory, fakes, noops, panics, wrapup.

Update README + architecture.mmd with goals feature.

README.md

@@ -18,6 +18,7 @@ A self-hosted, server-side rendered movie logging system with a full REST API. B
 - User profiles — display name, bio, avatar, banner, custom profile fields; editable via HTML settings page or REST API
 - Jellyfin/Plex auto-import — media server sends a webhook on playback stop, movies land in a watch queue; review and confirm with a rating to create diary entries; per-user webhook tokens with SHA-256 auth; setup UI at `/settings/integrations`
 - Annual Wrap-Up — Spotify Wrapped for movies: per-user and instance-wide year-in-review with stats (top directors, actors, genres, rating distribution, watch time, rewatches, budget analysis), shareable HTML page at `/wrapups/{user_id}/{year}`, downloadable MP4 video with branded slides; admin-triggered or auto-generated in January
+- Goals — set a "watch N movies in YEAR" target with a progress bar; progress computed from existing reviews (backwards compatible); per-user federation toggle in settings; displayed on profile (SPA: interactive with create/edit/delete, classic HTML: read-only glassmorphic card)
 - CSV and JSON diary export
 - File importer: upload CSV, TSV, JSON, or XLSX from any source (Letterboxd, IMDb, etc.), map columns to domain fields via a step-by-step wizard or REST API, save mapping profiles for repeat imports
 - REST API v1 (`/api/v1/`) with full feature parity with the HTML interface