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.

test_fetch_with_db.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+"""
+Test script to demonstrate the modified fetch_folder_emails function
+that uses database dates for intelligent email fetching.
+"""
+
+from src.database import SessionLocal, get_latest_email_date
+from src.zoho_client import ZohoClient
+
+
+def test_fetch_with_database():
+    """Test the modified fetch function with database integration."""
+
+    # Create database session
+    db = SessionLocal()
+
+    try:
+        # Create Zoho client
+        client = ZohoClient()
+
+        # Example account email (replace with actual)
+        account_email = client.email
+
+        print("=== Testing fetch_folder_emails with database integration ===")
+
+        # Check what's the latest date in database
+        latest_date = get_latest_email_date(db, account_email, "INBOX")
+        print(f"Latest email date in database: {latest_date}")
+
+        # Fetch emails using the new function
+        emails = client.fetch_folder_emails(
+            folder="INBOX",
+            max_results=10,
+            days_back=30,  # Fallback if no database date
+            db_session=db,
+            account_email=account_email,
+        )
+
+        print(f"Fetched {len(emails)} emails")
+
+        # Display first few emails
+        for i, email in enumerate(emails[:3]):
+            print(f"\nEmail {i + 1}:")
+            print(f"  Subject: {email.get('subject', 'No subject')}")
+            print(f"  From: {email.get('from', 'Unknown')}")
+            print(f"  Date: {email.get('date', 'Unknown')}")
+
+        client.close()
+
+    except Exception as e:
+        print(f"Error: {e}")
+    finally:
+        db.close()
+
+
+if __name__ == "__main__":
+    test_fetch_with_database()