clip-search: backlog drain + /photos/search endpoint

Wires the persistence layer for CLIP semantic search. The watcher's
per-tick drain encodes any image_exif row with a known content_hash
but no clip_embedding via Apollo (cap CLIP_BACKLOG_MAX_PER_TICK,
default 32). On a query, /photos/search encodes the text via Apollo
and reranks every stored embedding in-memory.

ExifDao additions:
- list_clip_unencoded_candidates — partial-index scan for drain
- backfill_clip_embedding — touches only the two new columns
- list_clip_index — dedup'd (hash, embedding) pull for search

clip_watch::run_clip_encoding_pass is the parallel fan-out — tokio
runtime per pass with CLIP_ENCODE_CONCURRENCY (default 4). No marker
rows for permanent failures yet; per-tick cap bounds the retry cost.

/photos/search params: q, limit, threshold (default 0.20), library,
model_version. Response is intentionally minimal (path + score) so
the frontend joins against existing photo-metadata routes lazily.

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

src/lib.rs

@@ -7,6 +7,8 @@ pub mod ai;
 pub mod auth;
 pub mod bin_progress;
 pub mod cleanup;
+pub mod clip_search;
+pub mod clip_watch;
 pub mod content_hash;
 pub mod data;
 pub mod database;