feat(provenance): /api/provenance image + post endpoints

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

backend/app/api/provenance.py

@@ -0,0 +1,33 @@
+"""Provenance API: image -> posts, post -> header payload.
+
+Separate from the gallery/tag APIs by design (provenance is its own
+system). Read-only.
+"""
+
+from quart import Blueprint, jsonify
+
+from ..extensions import get_session
+from ..services.provenance_service import ProvenanceService
+
+provenance_bp = Blueprint("provenance", __name__,
+                          url_prefix="/api/provenance")
+
+
+@provenance_bp.route("/image/<int:image_id>", methods=["GET"])
+async def image_provenance(image_id: int):
+    async with get_session() as session:
+        svc = ProvenanceService(session)
+        payload = await svc.for_image(image_id)
+    if payload is None:
+        return jsonify({"error": "not found"}), 404
+    return jsonify(payload)
+
+
+@provenance_bp.route("/post/<int:post_id>", methods=["GET"])
+async def post_provenance(post_id: int):
+    async with get_session() as session:
+        svc = ProvenanceService(session)
+        payload = await svc.for_post(post_id)
+    if payload is None:
+        return jsonify({"error": "not found"}), 404
+    return jsonify(payload)