feat: introduce flow_id with timestamp-based report versioning

Replace run_id with flow_id as the primary grouping concept (one flow =
one user analysis intent spanning scan + pipeline + portfolio). Reports
are now written as {timestamp}_{name}.json so load methods always return
the latest version by lexicographic sort, eliminating the latest.json
pointer pattern for new flows.

Key changes:
- report_paths.py: add generate_flow_id(), ts_now() (ms precision),
  flow_id kwarg on all path helpers; keep run_id / pointer helpers for
  backward compatibility
- ReportStore: dual-mode save/load — flow_id uses timestamped layout,
  run_id uses legacy runs/{id}/ layout with latest.json
- MongoReportStore: add flow_id field and index; run_id stays for compat
- DualReportStore: expose flow_id property
- store_factory: accept flow_id as primary param, run_id as alias
- runs.py / langgraph_engine.py: generate and thread flow_id through all
  trigger endpoints and run methods
- Tests: add flow_id coverage for all layers; 905 tests pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent_os/backend/services/langgraph_engine.py

@@ -694,9 +694,9 @@ class LangGraphEngine:
         date = params.get("date", time.strftime("%Y-%m-%d"))
         portfolio_id = params.get("portfolio_id", "main_portfolio")
 
+        store = create_report_store()
         flow_id = params.get("flow_id") or generate_flow_id()
-        store = create_report_store(flow_id=flow_id)
-        writer_store = store
+        writer_store = create_report_store(flow_id=flow_id)
 
         if phase == "analysts":
             # Full re-run

tradingagents/portfolio/dual_report_store.py

@@ -72,6 +72,11 @@ class DualReportStore:
         """The flow identifier set on this store, if any."""
         return self._local.flow_id
 
+    @property
+    def flow_id(self) -> str | None:
+        """The flow identifier set on this store, if any."""
+        return self._local.flow_id
+
     @property
     def run_id(self) -> str | None:
         """The run/flow identifier (flow_id takes precedence)."""

tradingagents/portfolio/mongo_report_store.py

@@ -101,6 +101,11 @@ class MongoReportStore:
         """The flow identifier set on this store, if any."""
         return self._flow_id
 
+    @property
+    def flow_id(self) -> str | None:
+        """The flow identifier set on this store, if any."""
+        return self._flow_id
+
     @property
     def run_id(self) -> str | None:
         """The run/flow identifier (flow_id takes precedence for backward compat)."""