docs: face detection system section + per-tick backlog drain env vars

CLAUDE.md gets an "Important Patterns → Face detection system" entry
covering the schema (why content_hash and not (library_id, rel_path)),
the file-watch hook + per-tick backlog drains, auto-bind on tag-name
match, manual-face create with EXIF orientation handling, and the
rerun-preserves-manual-rows contract. README's face section adds
the two new env vars (FACE_BACKLOG_MAX_PER_TICK and
FACE_HASH_BACKFILL_MAX_PER_TICK) shipped this cycle so operators
know they're tunable.

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

README.md

@@ -181,4 +181,12 @@ unset.
   via its single-worker GPU pool.
 - `FACE_DETECT_TIMEOUT_SEC` - reqwest client timeout per detect call
   [default: `60`]. CPU inference on a backlog can take many seconds.
+- `FACE_BACKLOG_MAX_PER_TICK` - Cap on the per-tick backlog drain (photos
+  with a content_hash but no face_detections row) [default: `64`]. Runs
+  every watcher tick regardless of quick-vs-full scan, so the unscanned
+  set drains independently of the file walk.
+- `FACE_HASH_BACKFILL_MAX_PER_TICK` - Cap on the per-tick content_hash
+  backfill (photos that were registered before the hash field was
+  populated retroactively) [default: `2000`]. Errors don't burn the cap;
+  only successful hashes count.