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/queue/memory.py

@@ -0,0 +1,45 @@
+"""In-memory ``TaskProducer`` — the default (local) ingest transport.
+
+A thin adapter over anyio's ``MemoryObjectSendStream`` so the local path is an
+explicit :class:`TaskProducer` (rather than relying on structural typing of a
+third-party class). Semantics are identical to using the stream directly:
+``send`` enqueues, ``clone`` yields an independent per-user handle, ``async
+with`` / ``aclose`` close the (cloned) send end so the processor pool's
+receivers observe end-of-stream.
+"""
+
+from __future__ import annotations
+
+from types import TracebackType
+from typing import TYPE_CHECKING
+
+from anyio.streams.memory import MemoryObjectSendStream
+
+if TYPE_CHECKING:
+    from ..scanner import DocumentTask
+
+
+class MemoryTaskProducer:
+    def __init__(self, stream: MemoryObjectSendStream[DocumentTask]):
+        self._stream = stream
+
+    async def send(self, task: DocumentTask, /) -> None:
+        await self._stream.send(task)
+
+    def clone(self) -> MemoryTaskProducer:
+        return MemoryTaskProducer(self._stream.clone())
+
+    async def __aenter__(self) -> MemoryTaskProducer:
+        await self._stream.__aenter__()
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc: BaseException | None,
+        tb: TracebackType | None,
+    ) -> None:
+        await self._stream.__aexit__(exc_type, exc, tb)
+
+    async def aclose(self) -> None:
+        await self._stream.aclose()