feat(persistence): PR8 T8.1 — ServiceAccountRow ORM + insert smoke

Adds deerflow/persistence/service_account/{__init__,model}.py with
ServiceAccountRow:
  - id: UUID36 PK
  - workspace_id: FK workspaces ON DELETE CASCADE
  - name: String(64)
  - role: String(16) default "member" (Stage 0 lone value; Stage 2 RBAC)
  - identity_mode: String(16) default "collapsed" — three states
    "collapsed" / "external_passthrough" / "both"; Stage 1 API key
    auth layer branches on this to decide whether each call writes
    an external_users row
  - status: String(16) default "active"
  - created_by: FK users ON DELETE RESTRICT (must hand off / delete
    SAs before removing their creator)
  - created_at / updated_at (UTC, onupdate)
  - Index idx_service_accounts_workspace (workspace_id, status)

Registers in deerflow/persistence/models/__init__.py so the engine's
side-effect import path picks it up for Base.metadata.create_all().

T8.1 test: insert_smoke writes a row through a session, reads it back,
asserts each column round-trips. SQLite ephemeral DB per test via
tmp_path, no Postgres required at this layer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/persistence/models/__init__.py

@@ -10,6 +10,7 @@ The actual ORM classes have moved to entity-specific subpackages:
 - ``deerflow.persistence.user``
 - ``deerflow.persistence.workspace``  (Stage 0 PR3)
 - ``deerflow.persistence.workspace_membership``  (Stage 0 PR3)
+- ``deerflow.persistence.service_account``  (Stage 0 PR8)
 
 ``RunEventRow`` remains in ``deerflow.persistence.models.run_event`` because
 its storage implementation lives in ``deerflow.runtime.events.store.db`` and
@@ -19,6 +20,7 @@ there is no matching entity directory.
 from deerflow.persistence.feedback.model import FeedbackRow
 from deerflow.persistence.models.run_event import RunEventRow
 from deerflow.persistence.run.model import RunRow
+from deerflow.persistence.service_account.model import ServiceAccountRow
 from deerflow.persistence.thread_meta.model import ThreadMetaRow
 from deerflow.persistence.user.model import UserRow
 from deerflow.persistence.workspace.model import WorkspaceRow
@@ -28,6 +30,7 @@ __all__ = [
     "FeedbackRow",
     "RunEventRow",
     "RunRow",
+    "ServiceAccountRow",
     "ThreadMetaRow",
     "UserRow",
     "WorkspaceMembershipRow",