refactor: migrate asyncio to anyio for consistent structured concurrency

Replace asyncio primitives with anyio equivalents throughout the codebase
to establish a single async pattern. This provides better structured
concurrency with automatic cancellation on errors and aligns with the
pytest anyio configuration.

Changes:
- hybrid.py: Replace asyncio.gather() with anyio task groups
- token_broker.py: Replace asyncio.Lock() with anyio.Lock()
- storage.py: Replace asyncio.run() with anyio.run()
- app.py: Replace tg.start_soon() with await tg.start() for task status
- processor.py: Add task_status parameter for structured startup
- scanner.py: Add task_status parameter for structured startup
- CLAUDE.md: Update async/await patterns guidance

The change from start_soon() to await tg.start() enables proper task
initialization signaling, ensuring background tasks are ready before
proceeding. This follows anyio best practices for structured concurrency.

All 118 unit tests pass with the new implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/vector/processor.py

@@ -8,6 +8,7 @@ import time
 import uuid
 
 import anyio
+from anyio.abc import TaskStatus
 from anyio.streams.memory import MemoryObjectReceiveStream
 from httpx import HTTPStatusError
 from qdrant_client.models import FieldCondition, Filter, MatchValue, PointStruct
@@ -34,6 +35,8 @@ async def processor_task(
     shutdown_event: anyio.Event,
     nc_client: NextcloudClient,
     user_id: str,
+    *,
+    task_status: TaskStatus = anyio.TASK_STATUS_IGNORED,
 ):
     """
     Process documents from stream concurrently.
@@ -53,9 +56,13 @@ async def processor_task(
         shutdown_event: Event signaling shutdown
         nc_client: Authenticated Nextcloud client
         user_id: User being processed
+        task_status: Status object for signaling task readiness
     """
     logger.info(f"Processor {worker_id} started")
 
+    # Signal that the task has started and is ready
+    task_status.started()
+
     while not shutdown_event.is_set():
         try:
             # Get document with timeout (allows checking shutdown)