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).

src/lib/prisma.ts

@@ -1,37 +1,19 @@
 import { PrismaClient } from '@prisma/client'
+import { PrismaPg } from '@prisma/adapter-pg'
 
 const globalForPrisma = globalThis as unknown as {
   prisma: PrismaClient | undefined
 }
 
-// Detect database provider from environment (default to sqlite for local development)
-// Next.js automatically loads environment variables from .env, .env.development, .env.production
-const databaseProvider = process.env.DATABASE_PROVIDER || 'sqlite'
 const databaseUrl = process.env.DATABASE_URL
 
-// Create PrismaClient with appropriate adapter
+if (!databaseUrl) {
+  throw new Error('DATABASE_URL environment variable is required.')
+}
+
 const createPrismaClient = () => {
-  let client: PrismaClient
-  
-  if (databaseProvider === 'postgresql') {
-    // Validate DATABASE_URL is present for PostgreSQL
-    if (!databaseUrl) {
-      throw new Error(
-        'DATABASE_URL environment variable is required when DATABASE_PROVIDER is set to postgresql. ' +
-        'Current DATABASE_PROVIDER: ' + databaseProvider
-      )
-    }
-    
-    // Use PrismaPg adapter for PostgreSQL
-    const { PrismaPg } = require('@prisma/adapter-pg')
-    const adapter = new PrismaPg({ connectionString: databaseUrl })
-    client = new PrismaClient({ adapter })
-  } else {
-    // No adapter needed for SQLite
-    client = new PrismaClient()
-  }
-  
-  return client
+  const adapter = new PrismaPg({ connectionString: databaseUrl })
+  return new PrismaClient({ adapter })
 }
 
 export const prisma = globalForPrisma.prisma ?? createPrismaClient()