[ZhuchkaTriplesix]

Closes #1502

local_db.dart went from 1396 to 361 lines. LocalDb.instance keeps every public method with the same signature, so none of the ~190 importers change; it now delegates.

file	what it holds
local_db_schema.dart	LocalDbSchema: version, onCreate, onUpgrade (v1 to v13), the open pragmas, the profile id
local_db_models.dart	ConnectionRow, SqlQueryHistoryEntry, MutationAuditEntry, McpActivityEntry, MutationAuditSource and the caps (re-exported from local_db.dart)
folder_repository.dart	folders and the connection's folder
sql_query_history_repository.dart	the history and its per-bucket pruning (the insert counters moved here)
activity_log_repository.dart	mutation audit and MCP activity
app_settings_repository.dart	key-value settings and ER layouts
connection_repository.dart	connections with secrets and SSH config; tells the history repository when one is removed

Each repository takes the open-database function, so it runs on any Database, an in-memory one included. Method bodies were moved as they were. LocalDb keeps the open/close lifecycle, the file operations (path, backup, reset, WAL cleanup) and backupBeforeUpdate.

Tests: local_db_schema_test.dart creates a database fresh, upgrades a hand-built version 1 database (connections and folders survive) and a version 11 one (refusal column, ER layouts table), and checks the pragmas and the profile id. storage_repositories_test.dart runs four repositories on an in-memory database without LocalDb. The existing storage, MCP, database, connections, workspace and e2e tests (1614) pass locally.