fix(usage): derive API key last-used and bound stream memory

[mojomast]

Summary

• make last_used_at accurate in user/UI key listings by deriving from MAX(usage_events.timestamp) per key instead of relying on stale nullable column writes
• surface derived last-used timestamps in /ui/me API key table
• replace unbounded stream chunk accumulation with lightweight StreamUsageTracker metadata tracking for usage attribution and logging
• keep request accounting behavior unchanged while reducing stream memory growth risk

Verification

• PYTHONPATH=src .venv/bin/python -m py_compile src/proxy_app/usage_queries.py src/proxy_app/routers/user_api.py src/proxy_app/routers/ui.py src/proxy_app/stream_usage.py src/proxy_app/main.py tests/test_api_keys.py tests/test_stream_usage.py
• PYTHONPATH=src .venv/bin/pytest -q tests/test_api_keys.py tests/test_stream_usage.py tests/test_usage_attribution.py

---

Important

Improve API key last-used tracking and stream memory management with StreamUsageTracker and derived timestamps.

• Behavior:
  • Derive last_used_at for API keys from MAX(usage_events.timestamp) instead of a nullable column.
  • Display derived last-used timestamps in /ui/me API key table.
  • Replace unbounded stream chunk accumulation with StreamUsageTracker for metadata tracking in stream_usage.py.
• Functions:
  • Add fetch_api_key_last_used_map() in usage_queries.py to derive last-used timestamps.
  • Update streaming_response_wrapper() in main.py to use StreamUsageTracker.
• Templates:
  • Update me.html to show derived last-used timestamps.
• Tests:
  • Add test_list_api_keys_uses_derived_last_used_timestamp() in test_api_keys.py.
  • Add test_stream_usage_tracker_keeps_only_minimal_metadata() in test_stream_usage.py.

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[mojomast]

Superseded by updates pushed to #123 to keep all MVP changes in one review thread.