Fixes #5612, #5611

Root cause: PostgreSQL's jsonb_array_elements() throws errors when the
'tokens' key is missing or malformed, while SQLite/MySQL return NULL
gracefully. This caused:
- 502 errors when creating admin after v0.25.3 → v0.26.0 upgrade
- Settings not persisting and users unable to stay logged in

Changes to store/db/postgres/user_setting.go:
- Remove strict JSONB operations from GetUserByPATHash query
- Fetch all PERSONAL_ACCESS_TOKENS rows and filter in Go
- Skip malformed/invalid JSON rows with continue (error recovery)
- Match SQLite/MySQL's forgiving error handling

New integration tests (store/test/user_setting_test.go):
- TestUserSettingGetUserByPATHashNoTokensKey
- TestUserSettingGetUserByPATHashEmptyTokensArray
- TestUserSettingGetUserByPATHashWithOtherUsers

New PostgreSQL-specific tests (store/db/postgres/user_setting_test.go):
- TestGetUserByPATHashWithMissingData (comprehensive edge cases)
- TestGetUserByPATHashPerformance (100+ users)
- TestUpsertUserSetting (basic upsert)

Test coverage:
✅ Missing PERSONAL_ACCESS_TOKENS key
✅ Empty/malformed JSON data
✅ Multiple users with mixed valid/invalid data
✅ Performance with 100+ users
✅ Error recovery without crashes

Benefits:
- No database migration required (TEXT column works fine)
- Backward compatible with v0.25.3 upgrades
- Handles missing/corrupt data gracefully
- Consistent behavior across all database drivers