Feat/centralized error handling

[KevinMB0220]

Summary

• Introduces a typed error class hierarchy (AppError + 8 subclasses) replacing ad-hoc Error objects scattered across routes and services
• Adds a single errorHandler Express middleware that owns all error-to-HTTP mapping, structured logging, and Prisma error translation
• All routes now propagate errors via next(error) instead of responding inline, eliminating ~140 lines of duplicated error handling code

Changes

New files

• src/utils/errors.ts — AppError base class + ValidationError (400), AuthenticationError (401), AuthorizationError (403), NotFoundError (404), ConflictError (409), BusinessRuleError (422), ExternalServiceError (503), ConfigurationError (500)
• src/middleware/errorHandler.middleware.ts — Centralized error handler; maps AppError subclasses, translates Prisma errors (P2025→404, P2002→409), falls back to 500; exports asyncHandler wrapper
• src/tests/errorHandler.spec.ts — 18 tests covering all error classes and middleware HTTP mapping

Modified files

• src/middleware/validate.middleware.ts — Calls next(new ValidationError(...)) instead of responding directly
• src/services/round.service.ts — Throws ConflictError("...", "ACTIVE_ROUND_EXISTS") instead of ad-hoc error object
• src/routes/* (8 files) — All catch blocks use next(error); inline 404/401 guards use typed error classes
• src/index.ts — Replaces inline global error handler with errorHandler middleware
• src/docs/openapi.ts — ErrorResponse schema updated to document the full standardized shape

Error response contract

{
  "error": "ValidationError",
  "message": "walletAddress is required",
  "code": "VALIDATION_ERROR",
  "details": [{ "field": "walletAddress", "message": "walletAddress is required" }]
}

Error Class	HTTP	Code example
ValidationError	400	VALIDATION_ERROR
AuthenticationError	401	INVALID_SIGNATURE, CHALLENGE_EXPIRED
AuthorizationError	403	AUTHORIZATION_ERROR
NotFoundError	404	NOT_FOUND
ConflictError	409	ACTIVE_ROUND_EXISTS
BusinessRuleError	422	INVALID_ROUND_STATE
ExternalServiceError	503	EXTERNAL_SERVICE_ERROR
Prisma P2025	404	NOT_FOUND
Prisma P2002	409	CONFLICT
Unknown Error	500	INTERNAL_ERROR

Test plan

• npm run lint passes (no type errors)
• New errorHandler.spec.ts — 18 tests, all green
• Existing auth.routes.spec.ts — all tests pass (updated mocks + error field assertions)
• Existing notifications.routes.spec.ts — all tests pass
• Trigger validation error → verify { error: "ValidationError", code: "VALIDATION_ERROR", details: [...] }
• Trigger auth error → verify { error: "AuthenticationError", code: "INVALID_SIGNATURE" }
• Start duplicate round → verify 409 with code: "ACTIVE_ROUND_EXISTS"
• Hit unknown route → verify 404 with code: "NOT_FOUND"

Closes #92