fix(qdrant): verify collection existence in the readiness check

Why:
- The chunks collection is now created by an explicit deployment step
  (qdrant_bootstrap), not at startup, which means a process can boot against
  a healthy Qdrant that has no collection at all. /readyz's previous check
  only called get_collections(), so it reported ready in that state — the
  misconfiguration stayed invisible until the first upload failed with a 502
  after already paying for the MinIO write and embedding round trips.

Changes:
- ping_qdrant() now checks collection_exists(collection) instead of just
  reachability.

src/api/routers/health.py

@@ -23,7 +23,9 @@ async def readyz(request: Request, response: Response) -> dict[str, bool]:
     postgres_ready, minio_ready, qdrant_ready = await asyncio.gather(
         ping_postgres(resources.db_engine, timeout),
         ping_minio(resources.minio_client, timeout),
-        ping_qdrant(resources.qdrant_client, timeout),
+        ping_qdrant(
+            resources.qdrant_client, timeout, collection=resources.settings.qdrant.collection
+        ),
     )
 
     result = {

src/infrastructure/qdrant/client.py

@@ -9,10 +9,22 @@ def create_client(settings: QdrantSettings) -> AsyncQdrantClient:
     return AsyncQdrantClient(url=settings.url, api_key=settings.api_key)
 
 
-async def ping(client: AsyncQdrantClient, timeout: float) -> bool:
+async def ping(client: AsyncQdrantClient, timeout: float, *, collection: str) -> bool:
+    """Whether Qdrant is reachable **and** the `chunks` collection exists.
+
+    Reachability alone is not readiness here. The collection is created by a
+    deployment step (`python -m src.cli.qdrant_bootstrap`, see ADR-0001
+    "Collection provisioning"), so a process can boot against a healthy Qdrant
+    that has no collection at all. Without this check that misconfiguration
+    stays invisible until the first upload fails with a `502` — after the
+    request has already paid for the MinIO write and the embedding round trips.
+
+    This is the Qdrant analogue of an unapplied Alembic migration, and it
+    belongs in `/readyz` for the same reason: it is a dependency-readiness
+    condition, not a process-health one.
+    """
     try:
         async with asyncio.timeout(timeout):
-            await client.get_collections()
+            return await client.collection_exists(collection)
     except Exception:
         return False
-    return True