Release v0.2.5 backend audit and hardening - #3

Summary

• harden backend request, storage, provider, audio, session, control, and library boundaries
• correct Gemini, Stability, local provider, dashboard, launcher, and macOS lifecycle integrations
• remove dead code, bound queues/caches/metadata, and make multi-step writes failure-safe
• align runtime and packaging metadata for v0.2.5

Verification

• 156 Python tests
• Ruff, compileall, dependency and lock checks
• OpenAPI integrity (74 paths, 86 unique operation IDs)
• dashboard JavaScript syntax and smoke test
• shell syntax checks
• macOS Swift build