Switch inverted index from stale check to incremental updates

Register a hook with the indexer so that file add/remove events
incrementally maintain the inverted index, removing the need for
periodic staleness checks and cooldowns. Rebuild the index once on
startup via init_inverted_index().

backend/main.py

@@ -44,7 +44,7 @@ from backend.indexer import (
     remove_vault_from_index,
     add_vault_to_index,
 )
-from backend.search import search, get_all_tags, advanced_search, suggest_titles, suggest_tags
+from backend.search import search, get_all_tags, advanced_search, suggest_titles, suggest_tags, init_inverted_index
 from backend.image_processor import preprocess_images
 from backend.attachment_indexer import rescan_vault_attachments, get_attachment_stats
 from backend.vault_settings import (
@@ -379,14 +379,9 @@ async def _on_vault_change(events: list):
     Processes each event (create/modify/delete/move) and updates the index
     incrementally, then broadcasts SSE notifications.
     """
-    import backend.indexer as idx
     updated_vaults = set()
     changes = []
 
-    # Temporarily suppress per-file generation increments to coalesce them
-    # into a single increment at the end of the batch.
-    old_gen = idx._index_generation
-
     for event in events:
         vault_name = event["vault"]
         event_type = event["type"]
@@ -415,11 +410,6 @@ async def _on_vault_change(events: list):
         except Exception as e:
             logger.error(f"Error processing {event_type} event for {src}: {e}")
 
-    # Restore generation to only increment by 1 for the whole batch
-    # (unless it was already incremented by other operations)
-    if idx._index_generation > old_gen + 1:
-        idx._index_generation = old_gen + 1
-
     if changes:
         await sse_manager.broadcast("index_updated", {
             "vaults": list(updated_vaults),
@@ -520,6 +510,9 @@ async def lifespan(app: FastAPI):
         logger.info("Background indexing started")
         await build_index(_progress_cb)
 
+        # Build inverted index for search (one-time, then incremental)
+        init_inverted_index()
+
         # Start file watcher
         config = _load_config()
         watcher_enabled = config.get("watcher_enabled", True)