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>

frontend/src/lib/api-types.gen.ts

@@ -345,6 +345,23 @@ export interface paths {
         patch: operations["update_category_api_v1_categories__category_id__patch"];
         trace?: never;
     };
+    "/api/v1/chat/thread": {
+        parameters: {
+            query?: never;
+            header?: never;
+            path?: never;
+            cookie?: never;
+        };
+        get?: never;
+        put?: never;
+        post?: never;
+        /** Clear Thread */
+        delete: operations["clear_thread_api_v1_chat_thread_delete"];
+        options?: never;
+        head?: never;
+        patch?: never;
+        trace?: never;
+    };
     "/api/v1/exchange-rate/": {
         parameters: {
             query?: never;
@@ -1200,6 +1217,11 @@ export interface components {
             /** Name */
             name?: string | null;
         };
+        /** ClearThreadResponse */
+        ClearThreadResponse: {
+            /** Cleared */
+            cleared: boolean;
+        };
         /**
          * Currency
          * @enum {string}
@@ -3023,6 +3045,39 @@ export interface operations {
             };
         };
     };
+    clear_thread_api_v1_chat_thread_delete: {
+        parameters: {
+            query?: never;
+            header?: {
+                authorization?: string | null;
+            };
+            path?: never;
+            cookie?: {
+                ws_token?: string | null;
+            };
+        };
+        requestBody?: never;
+        responses: {
+            /** @description Successful Response */
+            200: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "application/json": components["schemas"]["ClearThreadResponse"];
+                };
+            };
+            /** @description Validation Error */
+            422: {
+                headers: {
+                    [name: string]: unknown;
+                };
+                content: {
+                    "application/json": components["schemas"]["HTTPValidationError"];
+                };
+            };
+        };
+    };
     current_rate_api_v1_exchange_rate__get: {
         parameters: {
             query?: never;