clip-search: migration + client + probe binary

Probe-phase scaffolding for CLIP semantic search. Adds the column
that will hold per-photo embeddings, the HTTP client to Apollo's
inference service, and a throwaway probe binary so we can eyeball
search-result quality on the live library before building the
persistence layer (backlog drain, /photos/search endpoint, UI).

- migrations/2026-05-14-000000_add_clip_embedding/ — adds
  image_exif.clip_embedding (BLOB) and clip_model_version (TEXT),
  plus a partial index on (clip_embedding IS NULL AND content_hash
  IS NOT NULL) for the future backfill drain.
- src/database/models.rs — extends ImageExif struct to match.
- src/ai/clip_client.rs — encode_image / encode_text / health,
  same Permanent/Transient/Disabled taxonomy as face_client.
- src/bin/probe_clip_search.rs — --query <q> --library N --limit M
  --top K. Encodes a sample and prints top-K cosine similarities.
  No DB writes.

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

src/ai/insight_chat.rs

@@ -2184,6 +2184,8 @@ mod tests {
             date_taken_source: None,
             original_date_taken: None,
             original_date_taken_source: None,
+            clip_embedding: None,
+            clip_model_version: None,
         });
         let out = resolve_date_taken_for_context(&exif, "Screenshot_2014-06-01.png");
         assert_eq!(out.as_deref(), Some("2021-08-15"));