feat(gallery): visual 'more like this' search (Phase 3 backend)

GalleryService.similar() ranks images by pgvector cosine distance to a source
image's precomputed SigLIP embedding — no query-time ML inference. Composes
with the Phase-1/2 scope filters (AND) but replaces the date sort (always
nearest-first, bounded top-N, no cursor). Returns None for a missing source
(→404), [] for a source with no embedding (video / pending ML); excludes self
and NULL-embedding rows. New GET /api/gallery/similar?similar_to=<id>&limit=N.
Image-detail payload gains has_embedding so the UI can hide the surface.

Alembic 0036 adds an HNSW vector_cosine_ops index on siglip_embedding (1152<2000
dims) so the search is sub-50ms ANN instead of a full scan; one-time ~30-60s
build over existing embeddings on deploy. Shared _gallery_images/_image_json
helpers de-dup the scroll/similar builders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/gallery.py

@@ -10,6 +10,21 @@ from ..services.gallery_service import GalleryService
 gallery_bp = Blueprint("gallery", __name__, url_prefix="/api/gallery")
 
 
+def _image_json(i):
+    """Serialize a GalleryImage for the scroll/similar list responses."""
+    return {
+        "id": i.id,
+        "sha256": i.sha256,
+        "mime": i.mime,
+        "width": i.width,
+        "height": i.height,
+        "created_at": i.created_at.isoformat(),
+        "posted_at": i.posted_at.isoformat() if i.posted_at else None,
+        "thumbnail_url": i.thumbnail_url,
+        "artist": i.artist,
+    }
+
+
 def _parse_date(raw):
     """Parse a YYYY-MM-DD query value to a UTC midnight datetime, or None.
     Raises ValueError (→ 400) on a malformed value."""
@@ -76,20 +91,7 @@ async def scroll():
 
     return jsonify(
         {
-            "images": [
-                {
-                    "id": i.id,
-                    "sha256": i.sha256,
-                    "mime": i.mime,
-                    "width": i.width,
-                    "height": i.height,
-                    "created_at": i.created_at.isoformat(),
-                    "posted_at": i.posted_at.isoformat() if i.posted_at else None,
-                    "thumbnail_url": i.thumbnail_url,
-                    "artist": i.artist,
-                }
-                for i in page.images
-            ],
+            "images": [_image_json(i) for i in page.images],
             "next_cursor": page.next_cursor,
             "date_groups": [
                 {"year": y, "month": m, "image_ids": ids} for y, m, ids in page.date_groups
@@ -98,6 +100,36 @@ async def scroll():
     )
 
 
+@gallery_bp.route("/similar", methods=["GET"])
+async def similar():
+    """Visual "more like this": images ranked by cosine distance to the
+    `similar_to` image's embedding. Composes with the scope filters (AND) but
+    ignores post_id and sort. Bounded top-N, no cursor."""
+    try:
+        similar_to = int(request.args["similar_to"])
+        limit = int(request.args.get("limit", "100"))
+        filters, _sort = _parse_filters()
+    except (KeyError, ValueError):
+        return jsonify({"error": "similar_to query param required"}), 400
+    # post_id is the exclusive post-detail view — not a similarity scope.
+    scope = {k: v for k, v in filters.items() if k != "post_id"}
+    async with get_session() as session:
+        svc = GalleryService(session)
+        try:
+            images = await svc.similar(image_id=similar_to, limit=limit, **scope)
+        except ValueError as exc:
+            return jsonify({"error": str(exc)}), 400
+    if images is None:
+        return jsonify({"error": "not found"}), 404
+    return jsonify(
+        {
+            "images": [_image_json(i) for i in images],
+            "next_cursor": None,
+            "date_groups": [],
+        }
+    )
+
+
 @gallery_bp.route("/timeline", methods=["GET"])
 async def timeline():
     try: