Implement audit logging for all workflow actions

[kael-developer]

Context

Problem
There is currently no trace of who created, started, paused, resumed, or modified a workflow, which hinders compliance, security auditing, and debugging.

Solution
Add an audit_logs table (user_id, action, workflow_id, timestamp, details). In the FastAPI app, create middleware that captures authenticated requests and logs the action. Extend CLI commands to emit structured audit entries using the same logger. Provide a secure API endpoint (/audit) to query logs with filters.

Value
Provides a complete, tamper‑evident trail of all user actions, satisfying audit requirements and simplifying incident investigations.

Implementation notes
Define the AuditLog model in the storage package. Implement a FastAPI dependency that records each request after successful authentication. Hook into Click command callbacks to write logs (using the same DB session). Add pagination and filtering to the /audit endpoint. Include unit and integration tests.
Priority: P1

Implementation summary

Lines 37-40 are stray duplicate struct fields not part of any type. Should be removed entirely. We'll replace them with nothing.

Closes #84