chore(quality): establish server coverage threshold

[pentaxis93]

Summary

Coverage reporting is now in place (#13) and shows the server at 36% coverage. A fail_under threshold should be set once the project has enough server-side tests to back a meaningful commitment. Until then, the threshold is 0 — coverage is tracked and visible but does not gate CI.

The current gaps:

• cli.py — 0% (376 lines)
• services/voice_daemon.py — 0% (232 lines)
• services/whisper.py — 26% (119 lines)
• core/rate_limit.py — 60%
• core/security.py — 58%
• db/session.py — 67%

The API layer is well-tested (84-100%).

Part of: #3

Scope

• Write server-side tests to reach the chosen threshold
• Set fail_under in server/pyproject.toml [tool.coverage.report]
• Ratchet the threshold to match the new baseline

Acceptance criteria

• Server test coverage reaches an agreed target (e.g., 60-70%)
• fail_under is set to that target in pyproject.toml
• CI enforces the threshold — coverage regressions fail the build

Dependencies

Depends on: #13 (coverage reporting must exist — now closed)
Related: #11 (parallel effort expanding client-side test coverage)