refactor: remove all SQLite code, standardize on PostgreSQL

- Remove database provider switching logic from prisma.ts and auth.ts
- Hardcode PostgreSQL as the only supported database
- Remove switch-database.js and use-dev-db.js scripts
- Remove Python utility scripts that used sqlite3 directly
- Update justfile to remove SQLite test targets
- Update package.json to remove db:switch script
- Update Dockerfile.ci-base to default to PostgreSQL
- Update .env.example to remove SQLite mention
- Update playwright.config.ts comments
- Update .gitignore to remove SQLite file patterns

This eliminates the root cause of test failures: the dev server and test
Prisma client were using different databases (PostgreSQL vs SQLite).

Dockerfile.ci-base

@@ -22,8 +22,7 @@ RUN echo "=== Bun Version ===" && bun --version && \
 WORKDIR /app
 
 # Set default environment variables
-ENV DATABASE_PROVIDER=sqlite
-ENV DATABASE_URL=file:./prisma/ci.db
+ENV DATABASE_PROVIDER=postgresql
 ENV BETTER_AUTH_SECRET=test-secret-key-for-ci-only
 ENV NODE_ENV=test