fix: address account valuation review findings

jarca_ledger_mcp/infrastructure/sqlite/connection.py

@@ -5,10 +5,15 @@ import sqlite3
 from contextlib import contextmanager
 from pathlib import Path
 from typing import Iterator
+from urllib.parse import quote
 
 DEFAULT_DB_PATH = Path.home() / ".local" / "share" / "jarca" / "ledger.sqlite3"
 
 
+class DatabaseNotInitializedError(RuntimeError):
+    """Raised when a read is requested before the ledger exists."""
+
+
 def resolve_db_path(db_path: str | None = None) -> Path:
     raw = db_path or os.getenv("JARCA_LEDGER_DB_PATH") or str(DEFAULT_DB_PATH)
     return Path(raw).expanduser().resolve()
@@ -23,6 +28,31 @@ def connect(db_path: str | None = None) -> sqlite3.Connection:
     return conn
 
 
+def connect_readonly(db_path: str | None = None) -> sqlite3.Connection:
+    """Open an existing ledger in SQLite URI read-only mode."""
+    path = resolve_db_path(db_path)
+    if not path.is_file():
+        raise DatabaseNotInitializedError("ledger database is not initialized")
+    uri = f"file:{quote(str(path), safe='/')}?mode=ro"
+    try:
+        conn = sqlite3.connect(uri, uri=True)
+    except sqlite3.Error as exc:
+        raise DatabaseNotInitializedError("ledger database is not initialized") from exc
+    conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA foreign_keys=ON")
+    return conn
+
+
+@contextmanager
+def read_db(db_path: str | None = None) -> Iterator[sqlite3.Connection]:
+    """Open an existing ledger without filesystem or transaction writes."""
+    conn = connect_readonly(db_path)
+    try:
+        yield conn
+    finally:
+        conn.close()
+
+
 @contextmanager
 def db(db_path: str | None = None) -> Iterator[sqlite3.Connection]:
     from .migrations import init_db