feat: add opt-in MCP decomposition hook points (design §10)

Adds the seven §10.2 hook-point modules + five env vars so Astrolabe Cloud can
offload document processing to the external document-processor / embedding
gateway. Purely additive: with every setting unset the server behaves exactly
as today, so self-hosters are unaffected (Deck #92).

Hook points (all default to current monolith behavior):
- config: EMBEDDING_PROVIDER, INGEST_MODE, STATUS_BACKEND,
  COLLECTION_METADATA_SOURCE, FACT_EVENT_EMITTER (+ supporting settings),
  validated in Settings.__post_init__ (fail-fast STATUS_BACKEND=local with
  INGEST_MODE=external); shared canonical.py.
- vector/payload_keys.py + acl_hash.py: cross-impl NAMESPACE/point_id (§2.2)
  and BLAKE2b-128 ACL hash (§11), pinned by fixtures shared with the
  document-processor repo.
- embedding/gateway_client.py: OpenAI-compatible GatewayProvider authenticating
  via M2M OIDC client-credentials (separate realm); manual-only registry entry.
- vector/collection_metadata.py: sentinel-point / API metadata source with env
  fallback.
- vector/queue/: hexagonal ingest producer ports + memory/NATS adapters
  (Postgres seam); INGEST_MODE=external publishes mcp.ingest.requested.{tenant}
  instead of the in-memory stream and skips the in-process processor pool. The
  lifespan becomes a composition root across both deployment branches.
- vector/queue/status.py: STATUS_BACKEND=bus subscriber feeding a StatusStore
  the vector-sync status endpoint reads.
- admin/payload_backfill.py: POST /api/v1/admin/payload-backfill (admin scope);
  processor writes the new payload keys; query-side ACL pre-filter gated behind
  ACL_PREFILTER_ENABLED (default off).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/oauth_sync.py

@@ -24,16 +24,14 @@ from dataclasses import dataclass, field
 
 import anyio
 from anyio.abc import TaskGroup, TaskStatus
-from anyio.streams.memory import (
-    MemoryObjectReceiveStream,
-    MemoryObjectSendStream,
-)
+from anyio.streams.memory import MemoryObjectReceiveStream
 from httpx import BasicAuth, HTTPStatusError
 
 from nextcloud_mcp_server.auth.storage import RefreshTokenStorage
 from nextcloud_mcp_server.client import NextcloudClient
 from nextcloud_mcp_server.config import get_settings
 from nextcloud_mcp_server.vector.processor import process_document
+from nextcloud_mcp_server.vector.queue.ports import TaskProducer
 from nextcloud_mcp_server.vector.scanner import DocumentTask, scan_user_documents
 
 logger = logging.getLogger(__name__)
@@ -101,7 +99,7 @@ async def get_user_client_basic_auth(
 
 async def user_scanner_task(
     user_id: str,
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     shutdown_event: anyio.Event,
     wake_event: anyio.Event,
     nextcloud_host: str,
@@ -330,7 +328,7 @@ oauth_processor_task = multi_user_processor_task
 async def _run_user_scanner_with_scope(
     user_id: str,
     cancel_scope: anyio.CancelScope,
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     shutdown_event: anyio.Event,
     wake_event: anyio.Event,
     nextcloud_host: str,
@@ -358,7 +356,7 @@ async def _run_user_scanner_with_scope(
 
 
 async def user_manager_task(
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     shutdown_event: anyio.Event,
     wake_event: anyio.Event,
     refresh_token_storage: "RefreshTokenStorage",