fix(06): CR-06 emit request_complete log line with duration_ms and status_code

Capture response status code in send_with_header closure via nonlocal.
After awaiting the app, emit a structured log line via
structlog.get_logger("docuvault.access").info("request_complete", ...)
so duration_ms is actually written to the log before contextvars are
cleared by the next request.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/main.py

@@ -109,8 +109,12 @@ class CorrelationIDMiddleware:
             method=scope.get("method", ""),
         )
 
+        _response_status: int = 0
+
         async def send_with_header(message: dict) -> None:
+            nonlocal _response_status
             if message["type"] == "http.response.start":
+                _response_status = message.get("status", 0)
                 headers = list(message.get("headers", []))
                 headers.append((b"x-correlation-id", correlation_id.encode()))
                 message = {**message, "headers": headers}
@@ -118,9 +122,12 @@ class CorrelationIDMiddleware:
 
         await self.app(scope, receive, send_with_header)
 
-        # Bind duration after the response so downstream loggers can emit it.
         duration_ms = (time.perf_counter_ns() - start_ns) / 1_000_000
         structlog.contextvars.bind_contextvars(duration_ms=round(duration_ms, 2))
+        structlog.get_logger("docuvault.access").info(
+            "request_complete",
+            status_code=_response_status,
+        )
 
 
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