fix(search): write cache under DATA_DIR, guard mkdir against read-only path (#3334)

services/search/cache.py set CACHE_DIR = services/cache (the source tree) and
mkdir'd it at import, unguarded. In Docker services/ is the read-only image
layer, so the mkdir fails at import (same class as the analytics bug #2366).
Move the cache under DATA_DIR/cache (writable on Docker and native) and wrap
the mkdir so an unwritable path disables disk cache instead of crashing import.

Part of #3331.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

services/search/cache.py

@@ -6,17 +6,23 @@ from datetime import datetime, timedelta
 from pathlib import Path
 from typing import Dict
 
+from core.constants import DATA_DIR
+
 logger = logging.getLogger(__name__)
 
 # Cache directories
-CACHE_DIR = Path(__file__).resolve().parent.parent / "cache"
+CACHE_DIR = Path(DATA_DIR) / "cache"
 SEARCH_CACHE_DIR = CACHE_DIR / "search"
 CONTENT_CACHE_DIR = CACHE_DIR / "content"
 CACHE_MAX_ENTRIES = 1000
 
-# Create cache directories
+# Create cache directories. Guarded so an unwritable path (e.g. a read-only
+# mount) degrades to no-disk-cache instead of crashing module import.
+try:
     SEARCH_CACHE_DIR.mkdir(parents=True, exist_ok=True)
     CONTENT_CACHE_DIR.mkdir(parents=True, exist_ok=True)
+except OSError as _e:
+    logger.warning("Search cache directory unavailable (%s); disk cache disabled", _e)
 
 # Track cache size for LRU eviction
 search_cache_index: Dict[str, datetime] = {}