feat(phash): import_settings.phash_threshold (migration 0006) exposed + validated in API

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

backend/app/api/settings.py

@@ -17,6 +17,7 @@ _EDITABLE_FIELDS = (
     "skip_single_color",
     "single_color_threshold",
     "single_color_tolerance",
+    "phash_threshold",
 )
 
 
@@ -34,6 +35,7 @@ async def get_import_settings():
         "skip_single_color": row.skip_single_color,
         "single_color_threshold": row.single_color_threshold,
         "single_color_tolerance": row.single_color_tolerance,
+        "phash_threshold": row.phash_threshold,
     })
 
 
@@ -43,6 +45,15 @@ async def update_import_settings():
     if not isinstance(body, dict):
         return jsonify({"error": "body must be a JSON object"}), 400
 
+    if "phash_threshold" in body and (
+        not isinstance(body["phash_threshold"], int)
+        or isinstance(body["phash_threshold"], bool)
+        or body["phash_threshold"] < 0
+    ):
+        return jsonify(
+            {"error": "phash_threshold must be a non-negative integer"}
+        ), 400
+
     async with get_session() as session:
         row = (
             await session.execute(select(ImportSettings).where(ImportSettings.id == 1))