feat: export complete workspace audit history

[pralav-25]

Add Export full history (CSV) to the Activity log and authenticated GET /api/activity/export. Downloads contain every event in the current workspace, newest first, while the on-screen timeline retains its 100-event limit. CSV serialization preserves notes and escapes formula-like messages.

Validation:

• Ruff passed; 28 Python tests and 4 frontend utility tests passed.
• Regression coverage checks histories longer than 100 events, CSV escaping, anonymous rejection, and workspace isolation.
• Browser smoke test signed into a fresh local demo and downloaded the audit CSV. PostgreSQL coverage is provided by the existing CI matrix.