feat: T27 AuthService DB session persistence - replace in-memory Map with SQLite sessions table

backend/src/database/schema.sql

@@ -83,6 +83,15 @@ CREATE TABLE IF NOT EXISTS blocks (
   FOREIGN KEY (drawing_id) REFERENCES drawings(id) ON DELETE CASCADE
 );
 
+-- ─── Sessions ──────────────────────────────────────
+CREATE TABLE IF NOT EXISTS sessions (
+  token TEXT PRIMARY KEY,
+  user_id TEXT NOT NULL,
+  expires_at INTEGER NOT NULL,
+  created_at TEXT NOT NULL DEFAULT (datetime('now')),
+  FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+);
+
 -- ─── Settings ───────────────────────────────────────
 CREATE TABLE IF NOT EXISTS settings (
   key TEXT PRIMARY KEY,