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).

.gitignore

@@ -53,10 +53,6 @@ next-env.d.ts
 /src/generated/prisma
 
 # database
-*.db
-*.db-journal
-prisma/dev.db*
-prisma/prisma/dev.db*
 playwright-report/
 .env.development
 .env.dev