Stop printing bearer tokens and API bodies to device logs

Problem

Every GET prints the Authorization header, which contains a live JWT, and the full decoded response body to the device log. Every POST prints its request body and response body. print() is not stripped in release builds, so this goes to logcat on Android and the unified log on iOS.

Anything that can read device logs then harvests a valid session token plus student PII: names, enrolment numbers, emails, phone numbers.

Where

• lib/services/generic/requestService.dart:11-12 (endpoint and header map, header contains Bearer <jwt>)
• lib/services/generic/requestService.dart:17 (full response body)
• lib/services/generic/requestService.dart:29-30, 35-36 (POST request and response bodies)
• 37 print() calls across lib/ in total

Expected

No credentials and no response bodies in any build's logs.

Fix sketch

• Delete the header and body prints outright.
• Route remaining diagnostics through a helper guarded by kDebugMode.
• Enable the avoid_print lint so they cannot come back.

Start with lib/services/generic/requestService.dart at the listed GET and POST print calls, then search lib/ for the remaining 37 print() calls. Remove credential and body logging, route any remaining diagnostics through a kDebugMode-guarded helper, and enable the avoid_print lint. Done means no credentials or response bodies appear in any build's logs.