feat: add session topic to Recent Sessions table

[arojunior]

Summary

• Extracts session titles from Claude Code's custom-title and ai-title JSONL records, with a fallback to the first user message (truncated to 100 chars, skipping IDE context blocks)
• Adds a Topic column to the Recent Sessions table in the dashboard and includes it in CSV exports
• Adds topic TEXT column to the sessions DB table with _ensure_column() migration for existing databases

Title priority: custom-title > ai-title > first user message

Closes #147

Test plan

• Run a full rescan (python cli.py scan) and verify the sessions table has a topic column populated
• Open the dashboard and confirm the Topic column appears in the Recent Sessions table
• Verify custom titles (user-set labels) take priority over AI-generated ones
• Verify sessions without title records fall back to the first user message
• Export sessions to CSV and confirm the Topic column is included
• Test schema migration: start with an existing usage.db (no topic column), run a scan, and verify the column is added

🤖 Generated with Claude Code