feat(memory): add db.py with connection and path resolution

paperforge/memory/__init__.py

@@ -0,0 +1,8 @@
+from __future__ import annotations
+
+from paperforge.memory.db import get_connection, get_memory_db_path
+
+__all__ = [
+    "get_connection",
+    "get_memory_db_path",
+]

paperforge/memory/db.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import sqlite3
+from pathlib import Path
+
+from paperforge.config import paperforge_paths
+
+
+def get_memory_db_path(vault: Path) -> Path:
+    """Return the absolute path to paperforge.db."""
+    paths = paperforge_paths(vault)
+    db_path = paths.get("memory_db")
+    if not db_path:
+        raise FileNotFoundError("memory_db path not configured")
+    return db_path
+
+
+def get_connection(db_path: Path, read_only: bool = False) -> sqlite3.Connection:
+    """Open a SQLite connection to paperforge.db with WAL mode.
+
+    Args:
+        db_path: Path to paperforge.db.
+        read_only: If True, open in read-only mode (for queries).
+    """
+    if read_only:
+        uri = "file:" + db_path.as_posix() + "?mode=ro"
+        conn = sqlite3.connect(uri, uri=True)
+    else:
+        db_path.parent.mkdir(parents=True, exist_ok=True)
+        conn = sqlite3.connect(str(db_path))
+    conn.row_factory = sqlite3.Row
+    if not read_only:
+        conn.execute("PRAGMA journal_mode=WAL;")
+        conn.execute("PRAGMA foreign_keys=ON;")
+    return conn