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/watcher.rs

@@ -268,6 +268,7 @@ pub fn watch_files(
     playlist_manager: Addr<VideoPlaylistManager>,
     preview_generator: Addr<video::actors::PreviewClipGenerator>,
     face_client: crate::ai::face_client::FaceClient,
+    clip_client: crate::ai::clip_client::ClipClient,
     excluded_dirs: Vec<String>,
     library_health: libraries::LibraryHealthMap,
 ) {
@@ -300,6 +301,14 @@ pub fn watch_files(
                  or APOLLO_API_BASE_URL to enable)"
             );
         }
+        if clip_client.is_enabled() {
+            info!("  CLIP semantic search: ENABLED");
+        } else {
+            info!(
+                "  CLIP semantic search: DISABLED (set APOLLO_CLIP_API_BASE_URL \
+                 or APOLLO_API_BASE_URL to enable)"
+            );
+        }
         {
             let libs = libs_lock.read().unwrap_or_else(|e| e.into_inner());
             for lib in libs.iter() {
@@ -463,6 +472,21 @@ pub fn watch_files(
                     );
                 }
 
+                // CLIP embedding backlog. Independent of face detection —
+                // drain runs whenever CLIP is enabled, even on deploys
+                // that don't have the face engine wired up. Mirrors the
+                // face drain shape (capped per tick, no-op when disabled).
+                if clip_client.is_enabled() {
+                    let context = opentelemetry::Context::new();
+                    backfill::process_clip_backlog(
+                        &context,
+                        lib,
+                        &clip_client,
+                        &exif_dao,
+                        &effective_excludes,
+                    );
+                }
+
                 // Date-taken backfill: drain rows whose canonical date is
                 // either unresolved or only fs_time-sourced. Independent
                 // of face detection — runs even on deploys that don't