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`.

main.py

@@ -1,13 +1,78 @@
-from fastapi import FastAPI
+import datetime
+import os
+from contextlib import asynccontextmanager
+from pathlib import Path
 
-app = FastAPI()
+from fastapi import FastAPI, HTTPException
+from pydantic import BaseModel, Field
+
+from services.ledger import LedgerService
+
+
+def _ledger_path() -> Path:
+    env_path = os.getenv("LEDGER_FILE")
+    if env_path:
+        return Path(env_path)
+
+    return Path.cwd() / "ledger.beancount"
+
+
+class QueryRequest(BaseModel):
+    query: str = Field(min_length=1)
+
+
+@asynccontextmanager
+async def lifespan(application: FastAPI):
+    ledger_path = _ledger_path()
+    service = LedgerService(ledger_path)
+    application.state.ledger_service = service
+
+    await service.start()
+
+    try:
+        yield
+    finally:
+        await service.stop()
+
+
+app = FastAPI(lifespan=lifespan)
 
 
 @app.get("/")
 async def root():
-    return {"message": "Hello World"}
+    return {"message": "Ledger API"}
 
 
-@app.get("/hello/{name}")
-async def say_hello(name: str):
-    return {"message": f"Hello {name}"}
+@app.get("/status")
+async def status():
+    service: LedgerService = app.state.ledger_service
+    snapshot = await service.snapshot()
+
+    return {
+        "ledger_path": str(service.ledger_path),
+        "last_loaded_at": snapshot.last_loaded_at.isoformat() + "Z",
+        "error_count": len(snapshot.errors),
+        "errors": [str(err) for err in snapshot.errors],
+    }
+
+
+@app.get("/accounts")
+async def accounts(on_date: datetime.date | None = None):
+    service: LedgerService = app.state.ledger_service
+    return await service.account_balances(on_date)
+
+
+@app.get("/journal")
+async def journal(limit: int | None = None):
+    service: LedgerService = app.state.ledger_service
+    return await service.journal_entries(limit)
+
+
+@app.post("/query")
+async def query(payload: QueryRequest):
+    service: LedgerService = app.state.ledger_service
+
+    try:
+        return await service.run_query(payload.query)
+    except Exception as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc