feat(db): Bun.SQL connection and portable migrations

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/contracts/src/server.ts

@@ -14,8 +14,7 @@ export type JobStatus = "queued" | "running" | "done" | "failed";
 export interface JobRecord { id: string; kind: string; status: JobStatus; progress: number; message: string | null; payload: unknown; result: unknown; error: string | null; createdAt: string; updatedAt: string }
 export interface JobQueue { enqueue(kind: string, payload: unknown): Promise<string>; get(id: string): Promise<JobRecord | null>; list(kind?: string): Promise<JobRecord[]> }
 export interface AuthService { userFromRequest(req: Request): Promise<AuthUser | null> }
-// Placeholder until Task 7 (@stc/db) exists.
-type Db = unknown;
+import type { Db } from "@stc/db";
 export interface ServerContext {
   config: AppConfig; db: Db; courses: CourseRepository;
   jobs: JobQueue; auth: AuthService;