feat: migrate backend logging from Winston to Pino with correlation IDs (refs #82)

- Replace Winston with Pino using API-compatible wrapper
- Add LOG_LEVEL env var support with validation and fallback
- Add correlation ID middleware (X-Request-Id from Traefik or UUID)
- Configure PostgreSQL logging env vars (POSTGRES_LOG_STATEMENT, POSTGRES_LOG_MIN_DURATION)
- Configure Redis loglevel via command args

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/package.json

@@ -40,7 +40,7 @@
     "resend": "^3.0.0",
     "stripe": "^20.2.0",
     "tar": "^7.4.3",
-    "winston": "^3.17.0",
+    "pino": "^9.6.0",
     "zod": "^3.24.1"
   },
   "devDependencies": {