feat: record upstream response headers in debug traces

[github-actions]

Closes #444

Records upstream provider HTTP response headers into the debug trace log when tracing is enabled, and surfaces them on the Debug Logs page.

Backend

• Added responseHeaders column to the debug_logs schema (SQLite + Postgres).
• Dispatcher now extracts response headers from the upstream fetch response for both streaming and non-streaming paths and stores them via DebugManager.addResponseHeaders().
• DebugManager persists responseHeaders in the in-memory log record and flushes it to the DB via UsageStorageService.

Frontend

• Added a Response Headers accordion panel on the Debug Logs detail view (yellow header, collapsible, with fold/copy actions like the other panels).
• Included responseHeaders in the Copy All JSON export and the Download payload.

Tests

• All existing backend tests pass (49 files, 404 tests).
• Formatting verified via bun run format:check.
• Frontend build succeeds.

[micookie2]

I have another solution: record the request and response headers and status codes. I'll submit it later. @mcowger