fix: make photo retention and exclusion mutually exclusive

pigway_frame_flow/core/photo_preferences.py

@@ -1,5 +1,17 @@
 """Persistent selection preferences scoped to a library and its stable item ID."""
 
+from pigway_frame_flow.errors import ErrorCode as E
+from pigway_frame_flow.errors import FlowError
+
+
+def ensure_compatible(starred, excluded):
+    if starred and excluded:
+        raise FlowError(
+            E.CONFLICT,
+            "Starred photos cannot be excluded; clear the existing mark first",
+            409,
+        )
+
 
 def flags(db, source_id, item_id):
     row = db.one(
@@ -11,6 +23,16 @@ def flags(db, source_id, item_id):
 
 def set_flags(db, source_id, item_id, *, starred=None, excluded=None):
     with db.transaction() as connection:
+        current = flags(db, source_id, item_id)
+        if source_id == "@assets":
+            meta = db.one(
+                "SELECT starred FROM asset_metadata WHERE asset_id=?", (item_id,)
+            )
+            current["starred"] = bool(meta and meta["starred"])
+        ensure_compatible(
+            current["starred"] if starred is None else starred,
+            current["excluded"] if excluded is None else excluded,
+        )
         connection.execute(
             "INSERT INTO photo_preferences(source_id,item_id,starred,excluded) VALUES(?,?,?,?) ON CONFLICT(source_id,item_id) DO UPDATE SET starred=coalesce(?,starred),excluded=coalesce(?,excluded)",
             (