refactor(api): Split DatabaseService into repository pattern.

Split the monolithic DatabaseService (1,492 lines) into focused repository
modules using the repository pattern for better maintainability and
separation of concerns.

Changes:
- Create new repositories/ directory with 5 focused repositories:
  - TransactionRepository: transaction data operations (264 lines)
  - AccountRepository: account data operations (128 lines)
  - BalanceRepository: balance data operations (107 lines)
  - MigrationRepository: all database migrations (629 lines)
  - SyncRepository: sync operation tracking (132 lines)
  - BaseRepository: shared database connection logic (28 lines)

- Refactor DatabaseService into a clean facade (287 lines):
  - Delegates data access to repositories
  - Maintains public API (no breaking changes)
  - Keeps data processors in service layer
  - Preserves require_sqlite decorator

- Update tests to mock repository methods instead of private methods
- Fix test references to internal methods (_persist_*, _get_*)

Benefits:
- Clear separation of concerns (one repository per domain)
- Easier maintenance (changes isolated to specific repositories)
- Better testability (repositories can be mocked individually)
- Improved code organization (from 1 file to 7 focused files)

All 114 tests passing.

tests/unit/test_configurable_paths.py

@@ -120,12 +120,10 @@ class TestConfigurablePaths:
                 "iban": "TEST_IBAN",
             }
 
-            # Use the internal balance persistence method since the test needs direct database access
+            # Use the public balance persistence method
             import asyncio
 
-            asyncio.run(
-                database_service._persist_balance_sqlite("test-account", balance_data)
-            )
+            asyncio.run(database_service.persist_balance("test-account", balance_data))
 
             # Retrieve balances
             balances = asyncio.run(