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.

cli/main.py

@@ -926,7 +926,7 @@ def format_tool_args(args, max_length=80) -> str:
         return result[:max_length - 3] + "..."
     return result
 
-def run_analysis():
+def run_analysis(checkpoint: bool = False):
     # First get all user selections
     selections = get_user_selections()
 
@@ -943,6 +943,7 @@ def run_analysis():
     config["openai_reasoning_effort"] = selections.get("openai_reasoning_effort")
     config["anthropic_effort"] = selections.get("anthropic_effort")
     config["output_language"] = selections.get("output_language", "English")
+    config["checkpoint_enabled"] = checkpoint
 
     # Create stats callback handler for tracking LLM/tool calls
     stats_handler = StatsCallbackHandler()
@@ -1197,8 +1198,23 @@ def run_analysis():
 
 
 @app.command()
-def analyze():
-    run_analysis()
+def analyze(
+    checkpoint: bool = typer.Option(
+        False,
+        "--checkpoint",
+        help="Enable checkpoint/resume: save state after each node so a crashed run can resume.",
+    ),
+    clear_checkpoints: bool = typer.Option(
+        False,
+        "--clear-checkpoints",
+        help="Delete all saved checkpoints before running (force fresh start).",
+    ),
+):
+    if clear_checkpoints:
+        from tradingagents.graph.checkpointer import clear_all_checkpoints
+        n = clear_all_checkpoints(DEFAULT_CONFIG["data_cache_dir"])
+        console.print(f"[yellow]Cleared {n} checkpoint(s).[/yellow]")
+    run_analysis(checkpoint=checkpoint)
 
 
 if __name__ == "__main__":