fix(pairing): add SQLite persistence, fix config defaults, align with plan
- Add SQLite persistence to DeviceRegistry (backed by rusqlite)
- Rename config fields: ttl_secs -> code_ttl_secs, max_pending -> max_pending_codes, max_attempts -> max_failed_attempts
- Update defaults: code_length 6 -> 8, ttl_secs 300 -> 3600, max_pending 10 -> 3
- Add attempts tracking to PendingPairing struct
- Add token_hash() and authenticate_and_hash() to PairingGuard
- Fix route paths: /api/pairing/submit -> /api/pair, /api/devices/{id}/rotate -> /api/devices/{id}/token/rotate
- Add QR code placeholder to Pairing.tsx
- Pass workspace_dir to DeviceRegistry constructor
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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