feat(tenant): add tenant_domains allowlist and /v1/domains management API

Why:
- Domain values are denormalized into every Qdrant point payload. Without
  validation, an unregistered or typo'd domain (e.g. "fier" for "fire")
  silently creates a new partition that retrieval never queries — the file
  ends up invisible rather than rejected. Tenants also need independently
  sized domain sets (one may run 14 insurance lines, another 6), which rules
  out an enum.

Changes:
- tenant_domains table (migration 41335d162de8) + repository, unique on
  (tenant_id, domain).
- src/application/domains/: ensure_domain_allowed() is the strict-allowlist
  check now run inside upload_source_file()'s first transaction, before any
  MinIO object, job row, or Qdrant point is written.
- /v1/domains (list/create/patch/disable/enable) gated on its own
  domains:read/domains:write scopes, deliberately separate from files:write
  so an upload key cannot create partitions. domain itself is immutable
  (denormalized into every point payload); only display_name is editable.
  Disable blocks new uploads without touching already-indexed points.

Impact:
- BREAKING: POST /v1/files now rejects any domain without an active
  tenant_domains row (400, unknown_domain). A domain must be created via
  POST /v1/domains before the first upload to it.

src/application/files/upload.py

@@ -30,6 +30,7 @@ from anyio import CapacityLimiter, Semaphore, fail_after, to_thread
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
 from src.application.auth.context import AuthContext
+from src.application.domains import ensure_domain_allowed
 from src.application.files.errors import InvalidUploadError
 from src.application.files.models import UploadResult
 from src.application.files.storage_keys import source_file_object_key
@@ -118,6 +119,12 @@ async def upload_source_file(
 
     async with acquire_ingestion_slot(concurrency_limiter):
         async with sessionmaker() as session:
+            # Strict allowlist, checked inside txn A before anything is written
+            # (ADR-0009). An unregistered domain would otherwise create a new
+            # Qdrant partition silently, leaving the file invisible to
+            # retrieval rather than failing.
+            await ensure_domain_allowed(session, tenant_id=auth.tenant_id, domain=domain)
+
             existing = await source_files_repo.find_active_by_content_hash(
                 session,
                 tenant_id=auth.tenant_id,