fix: standardize error handling, redact sensitive log data, align response shapes

[Leothosine]

Summary

Standardizes error handling across the NestJS backend: redacts sensitive data from logs and aligns all error response shapes.

Changes

src/common/filters/http-exception.filter.ts

• Redacts Authorization, Cookie, Set-Cookie, X-Api-Key and other sensitive headers before logging
• Redacts sensitive body fields (password, token, secret, etc.) using the same key set as LoggingInterceptor
• Truncates bodies larger than 2,000 characters to [BODY_TOO_LARGE] to prevent log flooding
• Response shape now includes timestamp and path fields (was already included)

src/throttler-exception.filter.ts

• Added timestamp and path fields to the 429 response to match the shape produced by HttpExceptionFilter

test/app.e2e-spec.ts

• Registered HttpExceptionFilter in the test module via APP_FILTER
• Added a describe block that asserts the documented error response shape (statusCode, timestamp, path, message) on 400 responses
• Wired mock LoggingService so the filter can be exercised without real dependencies

closes #22

[MaryammAli]

LGTM
Thank you for your contribution