fix: code-review follow-ups (#464–#468)

[ZhuchkaTriplesix]

Summary

• bug(sql): injectSqlLimit must not rewrite LIMIT inside string literals #464: injectSqlLimit ignores LIMIT text inside string literals and dollar quotes
• bug(export): delete partial file when saveToFile streaming fails #465: delete partial export file when saveToFile fails mid-write
• bug(connections): PG schema tree must not show stale objects with error #466: PG schema tree clears object lists on load error (no stale + error)
• bug(theme): do not drop structural refresh while theme watcher in flight #467: theme folder watcher queues refresh while one is in flight and preserves structural flag
• bug(redis): clear keys-view error banner after successful TYPE/TTL batch #468: Redis keys view clears TYPE/TTL error banner on successful scan

Test plan

• flutter test test/core/database/sql_limit_test.dart
• flutter test test/core/theme/theme_folder_watcher_test.dart
• flutter test test/shared/data_export_service_test.dart
• Manual: export large table, force failure → no partial file left
• Manual: PG schema expand with broken connection → error, no stale children
• Manual: edit theme JSON rapidly while refresh slow → later edits applied
• Manual: Redis SCAN with TYPE/TTL failure then success → banner clears