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).

.env.example

@@ -11,7 +11,7 @@ DATABASE_URL=postgresql://euchre:euchrepassword@localhost:5432/euchre_camp
 # Shadow database for Prisma migrations (optional for PostgreSQL)
 DATABASE_SHADOW_URL=postgresql://euchre:euchrepassword@localhost:5432/euchre_camp_shadow
 
-# Database provider (postgresql, mysql, sqlite, etc.)
+# Database provider (postgresql)
 DATABASE_PROVIDER=postgresql
 
 # ============================================