fix(routes): require authentication on upload, process, and generate-sheet endpoints

[singhanurag0317-bit]

Fix: Require authentication on upload, process, and print routes

Closes #891

Problem

Three critical routes lacked authMiddleware, letting anyone upload files, trigger AI processing, or generate print sheets without being logged in:

• POST /api/upload (and /api/upload/batch)
• POST /api/process
• POST /api/print/generate-sheet

Change

Added authMiddleware to the handler chains:

• backend/src/routes/upload.routes.js — / and /batch
• backend/src/routes/image.routes.js — / and /process
• backend/src/routes/print.routes.js — /generate-sheet

The public GET /api/print/presets stays unauthenticated. /api/upload/batch is protected as well since it is the same unauthenticated file-upload vector.

Tests

New backend/src/__tests__/routeAuth.test.js (supertest integration):

• Each protected POST route returns 401 with "No authentication token provided" when no token is sent.
• GET /api/print/presets remains accessible without a token.

Full backend suite: 9 suites / 26 tests passing.

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