feat: Add PostgreSQL support and enhance admin dashboard

- Migrate all service files from hardcoded SQLite to dual SQLite/PostgreSQL support
- Add PostgreSQL service to docker-compose.yml
- Create database abstraction layer (database.py) with execute_query, row_to_dict helpers
- Update all 7 service files: account, rating, contact, ai_cost, quota, site_assistant, admin
- Add new admin endpoint /database-stats for table size and row count visualization
- Add database_type field to system health endpoint
- Update .env.example with proper PostgreSQL connection string

.env.example

@@ -41,6 +41,7 @@ UPLOAD_FOLDER=/tmp/uploads
 OUTPUT_FOLDER=/tmp/outputs
 FILE_EXPIRY_SECONDS=1800
 STORAGE_ALLOW_LOCAL_FALLBACK=true
+# Use DATABASE_PATH for SQLite (development) or DATABASE_URL for PostgreSQL (production)
 DATABASE_PATH=/app/data/dociva.db
 
 # CORS
@@ -64,8 +65,8 @@ STRIPE_PRICE_ID_PRO_YEARLY=
 SENTRY_DSN=
 SENTRY_ENVIRONMENT=production
 
-# PostgreSQL (production) — leave empty to use SQLite
-DATABASE_URL=
+# PostgreSQL (production)
+DATABASE_URL=postgresql://dociva:${POSTGRES_PASSWORD}@postgres:5432/dociva
 POSTGRES_DB=dociva
 POSTGRES_USER=dociva
 POSTGRES_PASSWORD=replace-with-strong-postgres-password