feat: initial CodeBoard monorepo scaffold

Turborepo monorepo with npm workspaces:
- apps/web: Next.js 14 frontend with Tailwind v4, SSE progress, doc viewer
- apps/worker: BullMQ job processor (clone → parse → LLM generate)
- packages/shared: TypeScript types
- packages/parser: Babel-based AST parser (JS/TS) + regex (Python)
- packages/llm: OpenAI/Anthropic provider abstraction + prompt pipeline
- packages/diagrams: Mermaid architecture & dependency graph generators
- packages/database: Prisma schema (PostgreSQL)
- Docker multi-stage build (web + worker targets)

All packages compile successfully with tsc and next build.

.env.example

@@ -0,0 +1,8 @@
+DATABASE_URL=postgresql://codeboard:codeboard@localhost:5432/codeboard
+REDIS_URL=redis://localhost:6379
+OPENAI_API_KEY=
+ANTHROPIC_API_KEY=
+GITHUB_CLIENT_ID=
+GITHUB_CLIENT_SECRET=
+NEXTAUTH_SECRET=
+NEXTAUTH_URL=http://localhost:3000