fix: skip subrequests and internal redirects in post_read_request

- FIX: Guard post_read_request hook with r->main and r->prev checks to
  avoid logging duplicate lines for subrequests and internal redirects
- DOC: Document subrequest/redirect filtering in architecture.yml

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

architecture.yml

@@ -31,6 +31,10 @@ context:
       Log as soon as the HTTP request is fully read to capture input-side data
       (client/server addresses, request line, headers) without waiting for
       application processing.
+    filters:
+      - Subrequests (r->main != NULL) are skipped.
+      - Internal redirects (r->prev != NULL) are skipped.
+      - Only the original client request is logged.
   logging_scope:
     coverage: all-traffic
     description: >