feat(#4): Implement Authentik OIDC authentication with BetterAuth

- Integrated BetterAuth library for modern authentication
- Added Session, Account, and Verification database tables
- Created complete auth module with service, controller, guards, and decorators
- Implemented shared authentication types in @mosaic/shared package
- Added comprehensive test coverage (26 tests passing)
- Documented type sharing strategy for monorepo
- Updated environment configuration with OIDC and JWT settings

Key architectural decisions:
- BetterAuth over Passport.js for better TypeScript support
- Separation of User (DB entity) vs AuthUser (client-safe subset)
- Shared types package to prevent FE/BE drift
- Factory pattern for auth config to use shared Prisma instance

Ready for frontend integration (Issue #6).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Fixes #4

apps/api/src/auth/decorators/current-user.decorator.ts

@@ -0,0 +1,6 @@
+import { createParamDecorator, ExecutionContext } from "@nestjs/common";
+
+export const CurrentUser = createParamDecorator((_data: unknown, ctx: ExecutionContext) => {
+  const request = ctx.switchToHttp().getRequest();
+  return request.user;
+});