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

backend/src/database/DatabaseInterface.ts

@@ -72,6 +72,13 @@ export interface DBUser {
   updated_at: string;
 }
 
+export interface DBSession {
+  token: string;
+  user_id: string;
+  expires_at: number;
+  created_at: string;
+}
+
 export interface DatabaseInterface {
   init(): Promise<void>;
   close(): void;
@@ -119,4 +126,10 @@ export interface DatabaseInterface {
   // Settings
   getSetting(key: string): DBSetting | null;
   setSetting(key: string, value: string): void;
+
+  // Sessions
+  createSession(data: Partial<DBSession>): DBSession;
+  getSession(token: string): DBSession | null;
+  deleteSession(token: string): boolean;
+  deleteExpiredSessions(): void;
 }