Add ledger management service and API integration

- Introduced `LedgerService` to handle ledger file operations, queries, and snapshots.
- Added API routes for journal entry retrieval, account balances, and custom queries using FastAPI.
- Updated project dependencies to include `ruff` for linting, along with its configuration.
- Integrated `lifespan` for managing the lifecycle of `LedgerService`.

pyproject.toml

@@ -7,5 +7,11 @@ dependencies = [
     "beancount>=3.2.0",
     "beanquery>=0.2.0",
     "fastapi>=0.128.0",
+    "ruff>=0.14.14",
     "uvicorn>=0.40.0",
 ]
+
+[dependency-groups]
+dev = [
+    "ruff>=0.14.14",
+]