Add summary logging with progress counters and aggregation

[CybotTM]

Problem

The migration process generates ~31M log lines for 68K work packages (~450 lines per WP). There's no periodic progress reporting or final summary aggregation.

Current State

• Individual INFO messages for every operation
• 155K+ warnings logged individually (e.g., BUG32 user not found)
• No periodic progress counters (e.g., "Completed 1000/68741 WPs")
• No final summary of warnings/errors

Requirements

1. Periodic progress logging (every N work packages, e.g., 1000):

   [INFO] Migration progress: 1000/68741 WPs (1.5%) - 10.2 WPs/sec
   

2. Warning aggregation - instead of 155K individual warnings:

   [INFO] Warning summary: 155182 user lookups used fallback (top: caroline.kuhn=24627, gitlab-ci=24084, Anonymous=17759)
   

3. Final migration summary:

   [SUCCESS] Migration complete:
   - Work packages: 68741 created, 0 failed
   - Journals: 850000 created
   - Warnings: 155182 (user fallbacks)
   - Errors: 2 (non-critical)
   - Duration: 11.3 hours (101 WPs/min)
   

Related

• ADR analysis identified excessive log verbosity
• 31M lines for 68K WPs is not sustainable for observability