refactor: introduce OpenCodeSupervisor for server lifecycle management

[chriswritescode-dev]

Summary

• Introduce OpenCodeSupervisor with lifecycle state machine (idle → starting → healthy/unhealthy → recovering → failed)
• Add operation locking to opencodeServerManager to prevent concurrent start/stop/restart/reload races
• Implement 4-step recovery pipeline: restart, debug capture, rollback last known good, seed default config
• Add health polling with configurable interval, threshold, and enable/disable toggle
• Wire supervisor into all route factories with graceful fallback to direct manager calls
• Add archiveBrokenConfig method to SettingsService for backup before rollback
• Add health watch configuration defaults and env vars (HEALTH_POLL_MS, HEALTH_FAILURE_THRESHOLD, HEALTH_WATCH_ENABLED)

Changes

• backend/src/services/opencode-supervisor.ts - New supervisor class with lifecycle management
• backend/src/services/opencode-single-server.ts - Operation locking and allowNested parameter
• backend/src/services/settings.ts - New archiveBrokenConfig method
• backend/src/index.ts - Wire up supervisor, update shutdown logic
• backend/src/routes/health.ts - Use supervisor lifecycle state for health checks
• backend/src/routes/oauth.ts - Delegate config reload to supervisor
• backend/src/routes/providers.ts - Delegate config reload to supervisor
• backend/src/routes/repos.ts - Delegate restart to supervisor
• backend/src/routes/settings.ts - Delegate reload/restart to supervisor (~20 call sites)
• shared/src/config/defaults.ts - Health watch config defaults
• shared/src/config/env.ts - Health watch env vars

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation

Checklist

• Code follows project style (no comments, named imports)
• TypeScript types are properly defined
• Tests added/updated (80% coverage target)
• pnpm lint passes locally
• pnpm typecheck passes locally