refactor(storage): decompose monolithic LocalDb into Repository pattern and MigrationRunner #1502

Problem

lib/core/storage/local_db.dart is currently 1,342 lines long and acts as a monolithic God Object. It combines multiple distinct domain responsibilities:

1. Schema Migrations: Schema creation and incremental upgrade logic across 13 versions (_onCreate, _onUpgrade, WAL / foreign key pragmas).
2. Connections CRUD: Querying, inserting, updating, soft/hard deleting connections, SSH tunneling configs, and secret migrations.
3. Folders Storage: Managing folder hierarchies, nesting, reordering, and folder-connection associations.
4. Query History: Recording, querying, filtering, and pruning SQL statement history.
5. Pinned Objects: Managing starred/pinned tables and collections per connection.

This coupling violates the Single Responsibility Principle, makes unit testing difficult without mounting a live SQLite database, and creates concurrency contention bottlenecks.

Scope

Decompose local_db.dart into dedicated single-responsibility repositories under lib/core/storage/:

• LocalDbMigrationRunner (or DatabaseMigrationService): Encapsulates onCreate, onUpgrade (v1 through v13) and pragma configurations.
• ConnectionRepository: Connection CRUD, SSH tunnel associations, environment tagging, secret store synchronization.
• FolderRepository: Hierarchical folder tree management and ordering.
• QueryHistoryRepository: Execution history storage, capping, and search.
• PinnedObjectsRepository: Managing pinned/favorited database objects.
• Keep LocalDb as a lightweight facade (or gradually migrate callers to the repositories directly) to preserve backward compatibility during the transition.

Acceptance

• local_db.dart reduced to a lightweight facade or removed in favor of repositories.
• Schema migration logic isolated in a testable migration runner with automated upgrade tests.
• Existing storage unit and integration tests remain 100% green.