feat: Implement async AI analysis for email threads

- Added `get_latest_email_date()` function in `database.py` to retrieve the most recent email date for a given account and folder.
- Enhanced `fetch_folder_emails()` in `zoho_client.py` to intelligently determine the start date for fetching emails based on the latest email date in the database.
- Introduced `analyze_and_update_threads_async()` for asynchronous analysis of email threads, allowing concurrent processing.
- Created a synchronous wrapper `analyze_and_update_threads()` for easier integration.
- Updated `fetch_emails()` to support database session and account email parameters.
- Added comprehensive documentation in `AI_ANALYSIS_GUIDE.md` detailing the new AI analysis functionality.
- Implemented tests for the new features, including `test_fetch_with_db.py`, `test_ai_analysis.py`, and `test_single_analysis.py`.
- Added error handling and logging improvements throughout the codebase.

reset_analysis.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+"""
+Reset AI analysis data for testing purposes.
+"""
+
+import os
+import sys
+
+# Add the src directory to the path
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "src"))
+
+from database import SessionLocal, Thread
+
+
+def main():
+    """Reset analysis data for all threads."""
+
+    db = SessionLocal()
+    try:
+        # Reset analysis data
+        threads = db.query(Thread).all()
+
+        for thread in threads:
+            thread.actionable = False
+            thread.ai_summary = None
+            thread.ai_confidence = None
+            thread.last_analyzed_at = None
+
+        db.commit()
+        print(f"Reset analysis data for {len(threads)} threads")
+
+    except Exception as e:
+        print(f"Error: {e}")
+        db.rollback()
+    finally:
+        db.close()
+
+
+if __name__ == "__main__":
+    main()