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/scanner.py

@@ -13,7 +13,6 @@ from typing import cast
 
 import anyio
 from anyio.abc import TaskStatus
-from anyio.streams.memory import MemoryObjectSendStream
 from qdrant_client import AsyncQdrantClient
 from qdrant_client.models import FieldCondition, Filter, MatchValue, Record
 
@@ -31,6 +30,7 @@ from nextcloud_mcp_server.vector.placeholder import (
     write_placeholder_point,
 )
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
+from nextcloud_mcp_server.vector.queue.ports import TaskProducer
 
 logger = logging.getLogger(__name__)
 
@@ -108,6 +108,11 @@ class DocumentTask:
     metadata: dict[str, int | str] | None = (
         None  # Additional metadata (e.g., board_id/stack_id for deck_card)
     )
+    # Change-detection token (Nextcloud etag). Used as the external ingest
+    # content_hash when present; the bus producer falls back to modified_at
+    # when it is None (deletes, or sources whose etag isn't threaded). Harmless
+    # in local mode — the in-process processor reads its own etag.
+    etag: str | None = None
 
 
 # Track documents potentially deleted (grace period before actual deletion)
@@ -179,7 +184,7 @@ async def get_last_indexed_timestamp(user_id: str) -> int | None:
 
 
 async def scanner_task(
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     shutdown_event: anyio.Event,
     wake_event: anyio.Event,
     nc_client: NextcloudClient,
@@ -233,7 +238,7 @@ async def scanner_task(
 
 async def scan_user_documents(
     user_id: str,
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     nc_client: NextcloudClient,
     initial_sync: bool = False,
 ):
@@ -339,6 +344,7 @@ async def scan_user_documents(
                         doc_type="note",
                         operation="index",
                         modified_at=modified_at,
+                        etag=note.get("etag", ""),
                     )
                 )
                 queued += 1
@@ -405,6 +411,7 @@ async def scan_user_documents(
                             doc_type="note",
                             operation="index",
                             modified_at=modified_at,
+                            etag=note.get("etag", ""),
                         )
                     )
                     queued += 1
@@ -740,7 +747,7 @@ async def scan_user_documents(
 
 async def scan_news_items(
     user_id: str,
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     nc_client: NextcloudClient,
     initial_sync: bool,
     scan_id: int,
@@ -928,7 +935,7 @@ async def scan_news_items(
 
 async def scan_deck_cards(
     user_id: str,
-    send_stream: MemoryObjectSendStream[DocumentTask],
+    send_stream: TaskProducer,
     nc_client: NextcloudClient,
     initial_sync: bool,
     scan_id: int,