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.

FETCH_MODIFICATION_SUMMARY.md

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+# Fetch Folder Emails Modification Summary
+
+## Changes Made
+
+### 1. Database Module (`database.py`)
+- Added `get_latest_email_date()` function to retrieve the most recent email date for a given account and folder
+- Added import for `datetime` to support the new function
+
+### 2. ZohoClient Module (`zoho_client.py`)
+- Modified `fetch_folder_emails()` to accept two new optional parameters:
+  - `db_session`: Database session for querying latest email dates
+  - `account_email`: Account email to identify which emails to check for latest date
+- Updated logic to:
+  1. Check database for latest email date if db_session and account_email are provided
+  2. Use latest date (minus 1 minute buffer) as start date for IMAP search
+  3. Fall back to `days_back` parameter if no database date is available
+- Updated `fetch_emails()` wrapper to support the new parameters
+- Enhanced documentation with detailed parameter descriptions
+
+### 3. App Module (`app.py`)
+- Modified email fetching calls to pass database session and account email
+- Reorganized code to create database session before fetching emails
+
+## How It Works
+
+### Before
+- Always fetched emails starting from X days back
+- No awareness of what emails were already in the database
+- Could result in fetching duplicate emails or missing recent ones
+
+### After
+- Intelligently determines start date based on database contents
+- If emails exist in database: starts from the latest email date
+- If no emails in database: falls back to `days_back` parameter
+- Adds 1-minute buffer to avoid missing emails with same timestamp
+- Reduces unnecessary email fetching and improves efficiency
+
+## Usage Examples
+
+### Basic Usage (Backwards Compatible)
+```python
+client = ZohoClient()
+emails = client.fetch_folder_emails(folder="INBOX", days_back=7)
+```
+
+### With Database Integration (Recommended)
+```python
+from database import SessionLocal
+
+db = SessionLocal()
+client = ZohoClient()
+emails = client.fetch_folder_emails(
+    folder="INBOX",
+    days_back=30,  # Fallback only
+    db_session=db,
+    account_email="user@example.com"
+)
+db.close()
+```
+
+## Benefits
+
+1. **Efficiency**: Only fetches new emails since last sync
+2. **Reliability**: Ensures no emails are missed between syncs
+3. **Backwards Compatibility**: Still works without database parameters
+4. **Flexibility**: Falls back gracefully when database is empty
+5. **Performance**: Reduces IMAP server load and processing time
+
+## Testing
+
+Use `test_fetch_with_db.py` to test the new functionality:
+```bash
+python test_fetch_with_db.py
+```
+
+The test script demonstrates:
+- Checking latest email date in database
+- Fetching emails with database integration
+- Displaying results