feat: Track and display documentation generation time

[myakove]

Problem

There's no visibility into how long documentation generation takes. Users can't tell if a generation is slow or fast, and there's no way to compare generation times across providers/models or projects.

Desired Behavior

• Track the total time for each documentation generation (from start to completion)
• Store the generation duration in the database alongside the project variant
• Display the generation time in the dashboard UI for each variant card
• Show the time in a human-readable format (e.g., "2m 34s", "45s")

Implementation Notes

• Add a generation_duration column to the projects table (integer, seconds)
• Record start time when generation begins in _generate_from_path() or _run_generation()
• Calculate and store duration when generation completes (status="ready")
• Display in the dashboard variant card alongside existing metadata (page count, last generated, commit SHA)
• Consider also showing generation time on the status/progress page while generating (elapsed time)

Deliverables

• Database column generation_duration added to projects table
• Duration tracked and stored on successful generation
• Duration displayed on dashboard variant cards
• Duration displayed on status page (elapsed while generating, final when ready)
• Migration for existing data (set NULL for existing projects)