feat: add pino-http request logging middleware

[Majormaxx]

Summary

Wires pino-http (already in dependencies) into app.ts before all routes, so every request produces a structured log line with method, URL, status code, and response time.

Changes — backend/src/app.ts

• Imported { pinoHttp } from "pino-http" (named export avoids the CJS/ESM default-import callable issue under NodeNext module resolution)
• Imported { logger } from "./logger.js"
• Added app.use(pinoHttp({ logger })) as the first middleware, before express.json()

Changes — backend/src/api/controllers/vaults.ts

• Fixed pre-existing lint error: renamed err → _err in two unused catch variables (required for CI lint to pass)

Acceptance criteria

• Each request produces a structured log line with method, url, statusCode, responseTime

Test plan

cd backend
npm run lint   # ✓ 0 errors
npm run build  # ✓ no TS errors
npm run test   # ✓ 55/55 tests pass

Closes #510

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