feat(api): add ETag caching, correlation ID tracing, standardized err…

[Dami24-hub]

Body:

## Summary of Changes

1. **ETag Conditional Responses (#985):**
   - Implemented `ETag` calculation and `If-None-Match` conditional handling returning `HTTP 304 Not Modified` for unchanged event and statistics payloads.

2. **Request Correlation ID Tracing (#986):**
   - Introduced middleware to validate client `X-Correlation-ID` headers or generate UUIDs, propagating correlation IDs into structured logs and outbound response headers.

3. **Standardized Machine-Readable Error Codes (#987):**
   - Established a unified JSON error payload structure using stable, documented error codes (`INVALID_PERMISSIONS`, `RESOURCE_NOT_FOUND`, etc.).

4. **Scoped API Credentials (#988):**
   - Implemented fine-grained permission scopes for integration API keys with server-side authorization enforcement and write-only key secret handling.

## Issue Resolution

Closes #985
Closes #986
Closes #987
Closes #988

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