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/domains/__init__.py

@@ -0,0 +1,27 @@
+"""Tenant-domain management and the upload-time allowlist check (ADR-0009)."""
+
+from src.application.domains.errors import (
+    DomainAlreadyExistsError,
+    DomainsError,
+    UnknownDomainError,
+)
+from src.application.domains.models import DomainResult
+from src.application.domains.service import (
+    create_domain,
+    ensure_domain_allowed,
+    list_domains,
+    set_domain_status,
+    update_domain,
+)
+
+__all__ = [
+    "DomainAlreadyExistsError",
+    "DomainResult",
+    "DomainsError",
+    "UnknownDomainError",
+    "create_domain",
+    "ensure_domain_allowed",
+    "list_domains",
+    "set_domain_status",
+    "update_domain",
+]

src/application/domains/errors.py

@@ -0,0 +1,21 @@
+"""Domain-management failures (ADR-0009). No HTTP knowledge here —
+`src/api/errors.py` maps these to status codes.
+"""
+
+
+class DomainsError(Exception):
+    """Base class for tenant-domain failures."""
+
+
+class UnknownDomainError(DomainsError):
+    """The upload named a domain the tenant has not registered, or one that is
+    disabled. Maps to `400`.
+
+    Rejecting is the whole point: an unrecognized `domain` would otherwise
+    create a new Qdrant partition silently, and a file in a partition nothing
+    queries is invisible rather than failed (ADR-0009).
+    """
+
+
+class DomainAlreadyExistsError(DomainsError):
+    """The tenant already has a domain with this key. Maps to `409`."""

src/application/domains/models.py

@@ -0,0 +1,16 @@
+"""Transport-agnostic results for the domain-management service."""
+
+import uuid
+from dataclasses import dataclass
+from datetime import datetime
+
+
+@dataclass(frozen=True)
+class DomainResult:
+    id: uuid.UUID
+    domain: str
+    display_name: str
+    status: str
+    metadata: dict[str, object]
+    created_at: datetime
+    updated_at: datetime

src/application/domains/service.py

@@ -0,0 +1,142 @@
+"""Tenant-domain management (ADR-0009).
+
+A tenant's domain set is per-tenant and varies in size — one may run 14
+insurance lines, another 6 — so it is data, not an enum.
+
+`ensure_domain_allowed` is the reason this package exists: it is the strict
+allowlist check the upload path runs before anything is written. Everything
+else here is the management surface the calling backend uses to populate that
+allowlist, under its own `domains:write` scope so an upload key cannot create
+partitions.
+
+`tenant_id` is always a required parameter taken from `AuthContext`, never from
+a request body (ADR-0002).
+"""
+
+import uuid
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+
+from src.application.domains.errors import DomainAlreadyExistsError, UnknownDomainError
+from src.application.domains.models import DomainResult
+from src.infrastructure.postgres.models.tenant_domain import TenantDomain
+from src.infrastructure.postgres.repositories import tenant_domains as repo
+
+
+async def _flush_and_refresh(session: AsyncSession, tenant_domain: TenantDomain) -> None:
+    """Materialize server-generated columns before the row leaves the session.
+
+    `updated_at` is `onupdate=func.now()`, so after an UPDATE its value lives in
+    the database, not in the instance. Reading it later would trigger a lazy
+    load outside any greenlet context (`MissingGreenlet`), so it is fetched here
+    while the session is still open.
+    """
+    await session.flush()
+    await session.refresh(tenant_domain)
+
+
+def _to_result(tenant_domain: TenantDomain) -> DomainResult:
+    return DomainResult(
+        id=tenant_domain.id,
+        domain=tenant_domain.domain,
+        display_name=tenant_domain.display_name,
+        status=tenant_domain.status,
+        metadata=tenant_domain.metadata_,
+        created_at=tenant_domain.created_at,
+        updated_at=tenant_domain.updated_at,
+    )
+
+
+async def ensure_domain_allowed(
+    session: AsyncSession, *, tenant_id: uuid.UUID, domain: str
+) -> None:
+    """Raise `UnknownDomainError` unless the tenant has this domain active.
+
+    Takes a session rather than a sessionmaker: the upload path calls this
+    inside its existing txn A, so the check costs no extra connection and
+    cannot pass and then go stale before the row is written.
+    """
+    tenant_domain = await repo.get(session, tenant_id=tenant_id, domain=domain)
+    if tenant_domain is None:
+        raise UnknownDomainError(
+            f"domain '{domain}' is not registered for this tenant; "
+            f"create it via POST /v1/domains before uploading to it"
+        )
+    if tenant_domain.status != "active":
+        raise UnknownDomainError(f"domain '{domain}' is disabled for this tenant")
+
+
+async def list_domains(
+    sessionmaker: async_sessionmaker[AsyncSession],
+    *,
+    tenant_id: uuid.UUID,
+    include_disabled: bool = False,
+) -> list[DomainResult]:
+    async with sessionmaker() as session:
+        found = await repo.list_for_tenant(
+            session, tenant_id=tenant_id, include_disabled=include_disabled
+        )
+    return [_to_result(item) for item in found]
+
+
+async def create_domain(
+    sessionmaker: async_sessionmaker[AsyncSession],
+    *,
+    tenant_id: uuid.UUID,
+    domain: str,
+    display_name: str,
+    metadata: dict[str, object] | None = None,
+) -> DomainResult:
+    async with sessionmaker() as session:
+        if await repo.get(session, tenant_id=tenant_id, domain=domain) is not None:
+            raise DomainAlreadyExistsError(f"domain '{domain}' already exists for this tenant")
+        created = repo.create(
+            session,
+            tenant_id=tenant_id,
+            domain=domain,
+            display_name=display_name,
+            metadata=metadata,
+        )
+        await session.commit()
+        return _to_result(created)
+
+
+async def update_domain(
+    sessionmaker: async_sessionmaker[AsyncSession],
+    *,
+    tenant_id: uuid.UUID,
+    domain: str,
+    display_name: str,
+) -> DomainResult:
+    """Only the label is mutable — see `repo.update_display_name`."""
+    async with sessionmaker() as session:
+        found = await repo.get(session, tenant_id=tenant_id, domain=domain)
+        if found is None:
+            raise UnknownDomainError(f"domain '{domain}' is not registered for this tenant")
+        repo.update_display_name(found, display_name=display_name)
+        await _flush_and_refresh(session, found)
+        await session.commit()
+        return _to_result(found)
+
+
+async def set_domain_status(
+    sessionmaker: async_sessionmaker[AsyncSession],
+    *,
+    tenant_id: uuid.UUID,
+    domain: str,
+    status: str,
+) -> DomainResult:
+    """Disable or re-enable a domain.
+
+    Disabling blocks new uploads and hides the domain from pickers. It does not
+    touch the points already indexed under it — removing those needs the
+    tenant-erasure workflow plan 001 defers.
+    """
+    async with sessionmaker() as session:
+        found = await repo.get(session, tenant_id=tenant_id, domain=domain)
+        if found is None:
+            raise UnknownDomainError(f"domain '{domain}' is not registered for this tenant")
+        repo.set_status(found, status=status)
+        await _flush_and_refresh(session, found)
+        await session.commit()
+        return _to_result(found)