feat: Add POST /flush-cache endpoint to clear all cached data and reset timers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/cache_utils.py

@@ -155,3 +155,19 @@ def write_cache(sheet_id: str, modified_time: str, data: list | dict) -> None:
         logger.info(f"Wrote cache for {sheet_id}")
     except Exception as e:
         logger.error(f"Failed to write cache {sheet_id}: {e}")
+
+def flush_cache():
+    """Delete all cache files and reset in-memory state. Returns count of deleted files."""
+    global _DRIVE_SERVICE
+    _LAST_CHECKED.clear()
+    _DRIVE_SERVICE = None
+
+    deleted = 0
+    if CACHE_DIR.exists():
+        for f in CACHE_DIR.glob("*_cache.json"):
+            f.unlink()
+            deleted += 1
+            logger.info(f"Deleted cache file: {f.name}")
+
+    logger.info(f"Cache flushed: {deleted} files deleted, timers reset")
+    return deleted