Assistant: persist the chat thread in Postgres

Back MAF's opt-in AG-UI snapshot persistence with a DB store so the
conversation survives page reloads and restarts:

- ChatThreadSnapshot: one upserted row per (scope, thread_id), JSONB
  messages/state/interrupt (migration 794baf50635b)
- PostgresSnapshotStore implements the AGUIThreadSnapshotStore protocol.
  It opens its own sessions (MAF saves during SSE streaming, after the
  request middleware has closed the ContextVar session) and sanitizes on
  save: camelCase toolCalls/toolCallId (the @ag-ui/client Zod schema
  strips snake_case, which would lose cards on replay), drop BFF ctx-*
  context messages, dedupe MAF's re-recorded multi-run turns, and drop
  orphaned tool results
- Endpoint mount gains snapshot_store + a constant scope resolver
  (single-user app; auth already enforced by agent_auth_and_session)
- DELETE /api/v1/chat/thread resets the conversation

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/models/models.py

@@ -537,3 +537,32 @@ class WaterMeterReading(WaterMeterReadingBase, table=True):
 class WaterMeterReadingRead(WaterMeterReadingBase):
     id: int
     created_at: datetime
+
+
+# --- Assistant Chat Thread ---
+
+
+class ChatThreadSnapshot(SQLModel, table=True):
+    """Latest AG-UI thread snapshot per (scope, thread_id).
+
+    Written by app/agent/snapshot_store.py: MAF overwrites the full
+    cumulative message list at each run end — one row per thread, not an
+    append log.
+    """
+
+    __table_args__ = (UniqueConstraint("scope", "thread_id"),)
+
+    id: Optional[int] = Field(default=None, primary_key=True)
+    scope: str = Field(index=True)
+    thread_id: str
+    messages: list = Field(
+        default_factory=list,
+        sa_column=Column(JSON_OR_JSONB, nullable=False, server_default="[]"),
+    )
+    state: Optional[dict] = Field(
+        default=None, sa_column=Column(JSON_OR_JSONB, nullable=True)
+    )
+    interrupt: Optional[list] = Field(
+        default=None, sa_column=Column(JSON_OR_JSONB, nullable=True)
+    )
+    updated_at: datetime = Field(default_factory=utcnow)