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).

playwright.config.ts

@@ -6,13 +6,13 @@ export default defineConfig({
   expect: {
     timeout: 5000
   },
-  // Run tests sequentially to avoid database conflicts with SQLite
+  // Run tests sequentially to avoid database conflicts
   fullyParallel: false,
   // Fail the build on CI if you accidentally left test.only in the source code.
   forbidOnly: !!process.env.CI,
   // Retry on CI only.
   retries: process.env.CI ? 2 : 0,
-  // Always run with 1 worker to avoid database conflicts with SQLite
+  // Always run with 1 worker to avoid database conflicts
   workers: 1,
   // Reporter to use
   reporter: 'html',