Centralize Database Layer into Monorepo Package (`@autonomy/db`)

Consolidate schema, migrations, and Drizzle config into @autonomy/db

1. Problem Statement

Currently, sites/student-site/src/lib/server/db/ and sites/teacher-site/src/lib/server/db/ contain duplicate schemas and separate drizzle.config.ts files. Both sites maintain independent drizzle/ migration directories (0001_perfect_charles_xavier.sql exists only on student-site). Running migrations from both repositories against a shared Turso database leads to race conditions, schema drift, and duplicate migrations.

2. Scope of Work

• Create a new monorepo package: packages/core/db (or packages/db) named @autonomy/db.
• Consolidate all Drizzle schema files (auth.schema.ts, schema.ts, relations) into the new package.
• Relocate migration generation (drizzle-kit generate), migrations execution (drizzle-kit migrate), and Drizzle Studio into @autonomy/db.
• Export a typed client and repository helpers consumable by both student-site and teacher-site.
• Update package.json workspaces and Turborepo pipeline tasks (turbo.json) so database build/check tasks run deterministically.

3. Acceptance Criteria

• sites/student-site and sites/teacher-site import schemas and db client from @autonomy/db.
• Only one canonical drizzle/ directory exists in the entire monorepo.
• Running pnpm --filter @autonomy/db db:migrate successfully applies all migrations cleanly to a blank SQLite/Turso database.
• Both applications pass pnpm check and pnpm build using the shared package.

Compare the duplicate database code under sites/student-site/src/lib/server/db/ and sites/teacher-site/src/lib/server/db/, including their drizzle.config.ts and drizzle/ directories. Then review packages/core/db or packages/db, the workspace package.json files, and turbo.json before choosing the package location. Done means both sites import the shared schema and client, one canonical drizzle/ directory remains, the db:migrate command works on a blank database, and both sites pass pnpm check and pnpm build.