Split main.rs: extract watcher loop into src/watcher.rs

main.rs drops from 1200 → 346 lines (90% smaller than the pre-branch
3542). What's left is the startup wiring it was always meant to be:
.env, migrations, AppState construction, route registration, server
bind. The four background-loop functions move into src/watcher.rs:

- watch_files (310 lines) — quick/full scan tick, per-library probe,
  backfill drain dispatch, missing-file scan, back-ref refresh,
  orphan GC.
- process_new_files (351 lines) — file walk → EXIF write →
  face-candidate build → HLS / preview-clip queueing →
  reconciliation. The "biggest untested chunk" from the earlier
  audit.
- cleanup_orphaned_playlists (167 lines) — separate slower-tick
  thread.
- playlist_needs_generation — small mtime-comparison helper.

Plus 4 unit tests for playlist_needs_generation (covers missing
playlist, newer playlist, newer video, video-missing-metadata
fallback).

main.rs's imports correspondingly shrink — Addr, HashSet, WalkDir,
Utc, InsertImageExif, and the bulk of video::actors all leave with
the watcher. CLAUDE.md updated to reflect the new module layout
(layered architecture box + module map for the face-detection
section).

cargo test --bin image-api: 329 passing (no regression).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -76,7 +76,10 @@ cargo run --bin cleanup_files -- --base-path /path/to/media --database-url ./dat
 ### Core Components
 
 **Layered Architecture:**
-- **HTTP Layer** (`main.rs`): Route handlers for images, videos, metadata, tags, favorites, memories
+- **Startup wiring** (`main.rs`): only ~350 lines — env load, migrations, AppState, route registration, server bind. Background jobs are kicked off here but defined elsewhere.
+- **HTTP Layer** (`handlers/{image,video,favorites}.rs`, `files.rs`, `tags.rs`, `faces.rs`, `memories.rs`, `ai/handlers.rs`): the route handlers, grouped by domain.
+- **Background loops** (`watcher.rs`): the file-watcher tick (`watch_files`, `process_new_files`) and the orphaned-playlist cleanup (`cleanup_orphaned_playlists`). Per-tick drains are factored into `backfill.rs` (`backfill_unhashed_backlog`, `backfill_missing_date_taken`, `backfill_missing_content_hashes`, `process_face_backlog`, `build_face_candidates`).
+- **Thumbnails** (`thumbnails.rs`): generation pipeline + the `IMAGE_GAUGE` / `VIDEO_GAUGE` Prometheus metrics.
 - **Auth Layer** (`auth.rs`): JWT token validation, Claims extraction via FromRequest trait
 - **Service Layer** (`files.rs`, `exif.rs`, `memories.rs`): Business logic for file operations and EXIF extraction
 - **DAO Layer** (`database/mod.rs`): Trait-based data access (ExifDao, UserDao, FavoriteDao, TagDao)
@@ -392,8 +395,8 @@ under 2021, not 2014 — on the theory that EXIF is more reliable than
 import-named filenames. The reverse case (no EXIF, filename has a
 date) is unchanged.
 
-The `backfill_missing_date_taken` drain (`src/main.rs`) runs every
-watcher tick alongside `backfill_unhashed_backlog`. It loads up to
+The `backfill_missing_date_taken` drain (`src/backfill.rs`) runs every
+watcher tick alongside `backfill_unhashed_backlog` (also `src/backfill.rs`). It loads up to
 `DATE_BACKFILL_MAX_PER_TICK` rows (default 500) where
 `date_taken IS NULL OR date_taken_source = 'fs_time'` (backed by the
 `idx_image_exif_date_backfill` partial index), runs the waterfall
@@ -504,9 +507,9 @@ ImageApi owns the face data; Apollo (sibling repo) hosts the insightface inferen
 
 **Why content_hash and not (library_id, rel_path):** ties face data to the bytes, not the path. A backup mount that copies files from the primary library naturally inherits the existing detections without re-running inference. This is the reference implementation of the multi-library data model — see "Multi-library data model" above.
 
-**File-watch hook** (`src/main.rs::process_new_files`): for each photo with a populated `content_hash`, check `FaceDao::already_scanned(hash)`; if not, send bytes (or embedded JPEG preview for RAW via `exif::extract_embedded_jpeg_preview`) to Apollo's `/api/internal/faces/detect`. K=`FACE_DETECT_CONCURRENCY` (default 8) parallel calls per scan tick; Apollo serializes them via its single-worker GPU pool. `face_watch.rs` is the Tokio orchestration layer.
+**File-watch hook** (`src/watcher.rs::process_new_files`): for each photo with a populated `content_hash`, check `FaceDao::already_scanned(hash)`; if not, send bytes (or embedded JPEG preview for RAW via `exif::extract_embedded_jpeg_preview`) to Apollo's `/api/internal/faces/detect`. K=`FACE_DETECT_CONCURRENCY` (default 8) parallel calls per scan tick; Apollo serializes them via its single-worker GPU pool. `face_watch.rs` is the Tokio orchestration layer.
 
-**Per-tick backlog drain** (also `src/main.rs`): two passes that run on every watcher tick regardless of quick-vs-full scan:
+**Per-tick backlog drain** (`src/backfill.rs`): two passes that run on every watcher tick regardless of quick-vs-full scan:
 - `backfill_unhashed_backlog` — populates `image_exif.content_hash` for photos that arrived before the hash field was retroactive. Capped by `FACE_HASH_BACKFILL_MAX_PER_TICK` (default 2000); errors don't burn the cap.
 - `process_face_backlog` — runs detection on photos that have a hash but no `face_detections` row. Capped by `FACE_BACKLOG_MAX_PER_TICK` (default 64). Selected via a SQL anti-join (`FaceDao::list_unscanned_candidates`); videos and EXCLUDED_DIRS paths filtered out client-side via `face_watch::filter_excluded` so they never reach Apollo.
 
@@ -521,6 +524,8 @@ ImageApi owns the face data; Apollo (sibling repo) hosts the insightface inferen
 Module map:
 - `src/faces.rs` — `FaceDao` trait + `SqliteFaceDao` impl, route handlers for `/faces/*`, `/image/faces/*`, `/persons/*`. Mirror of `tags.rs` layout.
 - `src/face_watch.rs` — Tokio orchestration for the file-watch detect pass; `filter_excluded` (PathExcluder + image-extension filter), `read_image_bytes_for_detect` (RAW preview fallback).
+- `src/backfill.rs` — per-tick drains (unhashed-hash, date_taken, face-backlog, etc.) called from `watcher::watch_files` and `watcher::process_new_files`.
+- `src/watcher.rs` — the watcher loop itself and `process_new_files` (file walk → EXIF write → face-candidate build).
 - `src/ai/face_client.rs` — HTTP client for Apollo's inference. Configured by `APOLLO_FACE_API_BASE_URL`, falls back to `APOLLO_API_BASE_URL`. Both unset → feature disabled, file-watch hook is a no-op.
 - `migrations/2026-04-29-000000_add_faces/` — schema.