Protect connections and define device-backup boundaries

[micahflee]

Closes #94

Summary

• store Bluesky connections in UUID-keyed iOS Keychain / Android Keystore-backed SecureStore entries
• migrate legacy AsyncStorage and runtime-database OAuth material with protected-write-first, restart-safe ordering
• keep disconnect and account deletion semantics distinct while removing credentials from runtime databases
• keep committed databases and media backup-eligible while assigning archive staging and reproducible caches to OS-excluded cache storage

Test plan

• npm run lint
• npm test -- --runInBand (631 passed, 3 skipped)
• npm run test:archive-contract
• generated Expo config validation for Android backup and SecureStore settings

Review

• completed independent Standards and Spec reviews against origin/main
• addressed terminology and duplicated deletion-protocol findings

(This was written by an LLM.)