feat: add LangGraph checkpoint resume for crash recovery (#594)

Long analyses can take many minutes; a crash or interruption forced users
to re-run from scratch and re-pay every LLM call.  This adds an opt-in
checkpoint layer backed by per-ticker SQLite databases so the graph
resumes from the last successful node.

How to use:
- CLI:    tradingagents analyze --checkpoint
- CLI:    tradingagents analyze --clear-checkpoints
- Python: config["checkpoint_enabled"] = True

Lifecycle:
- propagate() recompiles the graph with a SqliteSaver when enabled and
  injects a deterministic thread_id derived from ticker+date so the
  same ticker+date resumes while a different date starts fresh.
- On successful completion the per-thread checkpoint rows are cleared.
- The context manager is closed in a try/finally so a crash never
  leaks the SQLite connection or leaves the graph in checkpoint mode.

Storage: ~/.tradingagents/cache/checkpoints/<TICKER>.db
(override via TRADINGAGENTS_CACHE_DIR).

The checkpointer module is new (tradingagents/graph/checkpointer.py)
and the GraphSetup now returns the uncompiled workflow so it can be
recompiled with a saver when needed.

Adds langgraph-checkpoint-sqlite>=2.0.0 dependency. 3 new tests verify
the crash/resume cycle and that a different date starts fresh.

tests/test_checkpoint_resume.py

@@ -0,0 +1,147 @@
+"""Test checkpoint resume: crash mid-analysis, re-run resumes from last node."""
+
+import sqlite3
+import tempfile
+import unittest
+from pathlib import Path
+from typing import TypedDict
+
+from langgraph.checkpoint.sqlite import SqliteSaver
+from langgraph.graph import END, StateGraph
+
+from tradingagents.graph.checkpointer import (
+    checkpoint_step,
+    clear_checkpoint,
+    get_checkpointer,
+    has_checkpoint,
+    thread_id,
+)
+
+# Mutable flag to simulate crash on first run
+_should_crash = False
+
+
+class _SimpleState(TypedDict):
+    count: int
+
+
+def _node_a(state: _SimpleState) -> dict:
+    return {"count": state["count"] + 1}
+
+
+def _node_b(state: _SimpleState) -> dict:
+    if _should_crash:
+        raise RuntimeError("simulated mid-analysis crash")
+    return {"count": state["count"] + 10}
+
+
+def _build_graph() -> StateGraph:
+    builder = StateGraph(_SimpleState)
+    builder.add_node("analyst", _node_a)
+    builder.add_node("trader", _node_b)
+    builder.set_entry_point("analyst")
+    builder.add_edge("analyst", "trader")
+    builder.add_edge("trader", END)
+    return builder
+
+
+class TestCheckpointResume(unittest.TestCase):
+    def setUp(self):
+        self.tmpdir = tempfile.mkdtemp()
+        self.ticker = "TEST"
+        self.date = "2026-04-20"
+
+    def test_crash_and_resume(self):
+        """Crash at 'trader' node, then resume from checkpoint."""
+        global _should_crash
+        builder = _build_graph()
+        tid = thread_id(self.ticker, self.date)
+        cfg = {"configurable": {"thread_id": tid}}
+
+        # Run 1: crash at trader node
+        _should_crash = True
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            with self.assertRaises(RuntimeError):
+                graph.invoke({"count": 0}, config=cfg)
+
+        # Checkpoint should exist at step 1 (analyst completed)
+        self.assertTrue(has_checkpoint(self.tmpdir, self.ticker, self.date))
+        step = checkpoint_step(self.tmpdir, self.ticker, self.date)
+        self.assertEqual(step, 1)
+
+        # Run 2: resume — trader succeeds this time
+        _should_crash = False
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            result = graph.invoke(None, config=cfg)
+
+        # analyst added 1, trader added 10 → 11
+        self.assertEqual(result["count"], 11)
+
+    def test_clear_checkpoint_allows_fresh_start(self):
+        """After clearing, the graph starts from scratch."""
+        global _should_crash
+        builder = _build_graph()
+        tid = thread_id(self.ticker, self.date)
+        cfg = {"configurable": {"thread_id": tid}}
+
+        # Create a checkpoint by crashing
+        _should_crash = True
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            with self.assertRaises(RuntimeError):
+                graph.invoke({"count": 0}, config=cfg)
+
+        self.assertTrue(has_checkpoint(self.tmpdir, self.ticker, self.date))
+
+        # Clear it
+        clear_checkpoint(self.tmpdir, self.ticker, self.date)
+        self.assertFalse(has_checkpoint(self.tmpdir, self.ticker, self.date))
+
+        # Fresh run succeeds from scratch
+        _should_crash = False
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            result = graph.invoke({"count": 0}, config=cfg)
+
+        self.assertEqual(result["count"], 11)
+
+
+    def test_different_date_starts_fresh(self):
+        """A different date must NOT resume from an existing checkpoint."""
+        global _should_crash
+        builder = _build_graph()
+        date2 = "2026-04-21"
+
+        # Run with date1 — crash to leave a checkpoint
+        _should_crash = True
+        tid1 = thread_id(self.ticker, self.date)
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            with self.assertRaises(RuntimeError):
+                graph.invoke({"count": 0}, config={"configurable": {"thread_id": tid1}})
+
+        self.assertTrue(has_checkpoint(self.tmpdir, self.ticker, self.date))
+
+        # date2 should have no checkpoint
+        self.assertFalse(has_checkpoint(self.tmpdir, self.ticker, date2))
+
+        # Run with date2 — should start fresh and succeed
+        _should_crash = False
+        tid2 = thread_id(self.ticker, date2)
+        self.assertNotEqual(tid1, tid2)
+
+        with get_checkpointer(self.tmpdir, self.ticker) as saver:
+            graph = builder.compile(checkpointer=saver)
+            result = graph.invoke({"count": 0}, config={"configurable": {"thread_id": tid2}})
+
+        # Fresh run: analyst +1, trader +10 = 11
+        self.assertEqual(result["count"], 11)
+
+        # Original date checkpoint still exists (untouched)
+        self.assertTrue(has_checkpoint(self.tmpdir, self.ticker, self.date))
+
+
+if __name__ == "__main__":
+    unittest.main()