fix(cache): one cleanup thread per cache directory, not per manager (#453)

The display process ran three cleanup threads over one directory:

    14:22:59.954  display_controller        (the real manager)
    14:22:59.973  startup validation, run 1 (discarded)
    14:23:01.055  startup validation, run 2 (discarded)

Two of those managers existed only to read a directory path.
StartupValidator._validate_cache_directory built a whole CacheManager to
call get_cache_dir(), and validation runs twice -- once before the
plugin manager exists and again after. Each construction also probes
writability by writing and deleting .writetest on the card.

The discarded ones never went away. cleanup_loop closes over `self`, so
the thread keeps its manager alive: two objects that could never be
collected, waking every 24 hours to re-scan the same 9,000-file
directory. Nothing stopped them either -- stop_cleanup_thread had no
callers anywhere in the tree.

Two changes. The validator now takes the CacheManager the application
actually uses, which is also the more correct thing to validate; when
no caller supplies one it still builds its own, but stops the thread
afterwards. And CacheManager now tracks which directory it is sweeping,
so the second manager over a directory skips starting a thread at all.
That is the right granularity regardless of call sites: the sweep lists
a directory and deletes from it, so a second thread only duplicates the
scan. Ownership is released on stop, so a survivor can take over rather
than leaving the directory permanently unclaimed by a dead owner.

Measured directly, three managers over one directory: 3 threads before,
1 after.


Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/cache_manager.py

@@ -46,7 +46,21 @@ from src.cache.disk_cache import DateTimeEncoder  # noqa: F401 - deliberate re-e
 
 class CacheManager:
     """Manages caching of API responses to reduce API calls."""
-    
+
+    # Which cache directories already have a cleanup thread in this process.
+    #
+    # The sweep is directory-scoped work -- it lists a directory and deletes
+    # from it -- so one per directory is the right number no matter how many
+    # managers exist. Nothing enforced that before: every instance started its
+    # own, and because the loop closes over `self`, a discarded manager could
+    # never be collected and its thread woke to re-scan the same directory
+    # every 24 hours for the life of the process. Startup validation runs
+    # twice and built a throwaway manager each time, so a display process
+    # carried three threads for one cache.
+    _cleanup_owners: Dict[str, 'CacheManager'] = {}
+    _cleanup_owners_lock = threading.Lock()
+
+
     def __init__(self) -> None:
         # Initialize logger first
         self.logger: logging.Logger = get_logger(__name__)
@@ -718,11 +732,29 @@ class CacheManager:
             }
     
     def start_cleanup_thread(self) -> None:
-        """Start background thread for periodic disk cache cleanup."""
+        """Start background thread for periodic disk cache cleanup.
+
+        At most one thread per cache directory per process: the sweep is
+        directory-scoped, so a second one only duplicates the scan.
+        """
         if self._cleanup_thread and self._cleanup_thread.is_alive():
             self.logger.debug("Cleanup thread already running")
             return
-        
+
+        with CacheManager._cleanup_owners_lock:
+            owner = CacheManager._cleanup_owners.get(self.cache_dir)
+            if owner is not None and owner is not self:
+                thread = owner._cleanup_thread
+                if thread is not None and thread.is_alive():
+                    self.logger.debug(
+                        "Cleanup thread for %s already owned by another cache "
+                        "manager in this process; not starting a second",
+                        self.cache_dir)
+                    return
+                # The owner's thread died or was stopped -- take over.
+            CacheManager._cleanup_owners[self.cache_dir] = self
+
+
         def cleanup_loop():
             """Background loop that runs cleanup periodically."""
             self.logger.info("Disk cache cleanup thread started (interval: %d hours)", 
@@ -770,10 +802,17 @@ class CacheManager:
         Signals the thread to stop and waits for it to finish (with timeout).
         This allows for clean shutdown during testing or application termination.
         """
+        # Release ownership first and unconditionally, so a manager that never
+        # started a thread (or whose thread already exited) cannot keep the
+        # directory claimed and block a live manager from sweeping it.
+        with CacheManager._cleanup_owners_lock:
+            if CacheManager._cleanup_owners.get(self.cache_dir) is self:
+                del CacheManager._cleanup_owners[self.cache_dir]
+
         if not self._cleanup_thread or not self._cleanup_thread.is_alive():
             self.logger.debug("Cleanup thread not running")
             return
-        
+
         self.logger.info("Stopping disk cache cleanup thread...")
         self._cleanup_stop_event.set()  # Signal thread to stop