Implement ownership model for all cached content deletion

- cache_service.delete_content: Remove user's ownership link first,
  only delete actual file if no other owners remain

- cache_manager.discard_activity_outputs_only: Check if outputs and
  intermediates are used by other activities before deleting

- run_service.discard_run: Now cleans up run outputs/intermediates
  (only if not shared by other runs)

- home.py clear_user_data: Use ownership model for effects and media
  deletion instead of directly deleting files

The ownership model ensures:
1. Multiple users can "own" the same cached content
2. Deleting removes the user's ownership link (item_types entry)
3. Actual files only deleted when no owners remain (garbage collection)
4. Shared intermediates between runs are preserved

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/run_service.py

@@ -467,10 +467,13 @@ class RunService:
         username: str,
     ) -> Tuple[bool, Optional[str]]:
         """
-        Discard (delete) a run record.
+        Discard (delete) a run record and clean up outputs/intermediates.
 
-        Note: This removes the run record but not the output content.
+        Outputs and intermediates are only deleted if not used by other runs.
         """
+        import logging
+        logger = logging.getLogger(__name__)
+
         run = await self.get_run(run_id)
         if not run:
             return False, f"Run {run_id} not found"
@@ -480,6 +483,18 @@ class RunService:
         if run_owner and run_owner not in (username, actor_id):
             return False, "Access denied"
 
+        # Clean up activity outputs/intermediates (only if orphaned)
+        # The activity_id is the same as run_id
+        try:
+            success, msg = self.cache.discard_activity_outputs_only(run_id)
+            if success:
+                logger.info(f"Cleaned up run {run_id}: {msg}")
+            else:
+                # Activity might not exist (old runs), that's OK
+                logger.debug(f"No activity cleanup for {run_id}: {msg}")
+        except Exception as e:
+            logger.warning(f"Failed to cleanup activity for {run_id}: {e}")
+
         # Remove task_id mapping from Redis
         self.redis.delete(f"{self.task_key_prefix}{run_id}")
 
@@ -487,8 +502,7 @@ class RunService:
         try:
             await self.db.delete_run_cache(run_id)
         except Exception as e:
-            import logging
-            logging.getLogger(__name__).warning(f"Failed to delete run_cache for {run_id}: {e}")
+            logger.warning(f"Failed to delete run_cache for {run_id}: {e}")
 
         # Remove pending run if exists
         try: