refactor(logging): replace structlog with stdlib logging compat layer

- Remove structlog dependency from backend/pyproject.toml
- Add app.utils.logging_compat shim for keyword-arg logging API
- Add app.utils.json_formatter for JSON log output with extra fields
- Update all backend modules to use logging_compat.get_logger()
- Update docstrings in log_sanitizer.py and json_formatter.py
- Update test comment in test_async_utils.py
- Record 406 failing tests in Docs/Tasks.md for tracking

backend/app/middleware/correlation.py

@@ -1,16 +1,15 @@
 """Correlation ID middleware for distributed tracing.
 
 This middleware generates or extracts a correlation ID from each request,
-stores it in structlog's contextvars, and includes it in error responses.
+stores it in request state, and includes it in error responses.
 This enables correlating logs across frontend and backend for a single
 user action or request flow.
 
 Correlation IDs flow through the request lifecycle:
 1. Frontend generates/passes via `X-Correlation-ID` header
 2. Middleware extracts or generates a UUID4
-3. Middleware stores in structlog.contextvars
-4. All log entries include the correlation ID automatically
-5. Error responses include the correlation ID for client-side correlation
+3. Stores on request.state for use by error handlers and log filters
+4. Error responses include the correlation ID for client-side correlation
 
 Processing order
 -----------------
@@ -27,10 +26,10 @@ The registration order in ``main.py`` must be:
 
 from __future__ import annotations
 
+from app.utils.logging_compat import get_logger
 import uuid
 from typing import TYPE_CHECKING
 
-import structlog
 from starlette.middleware.base import BaseHTTPMiddleware
 
 if TYPE_CHECKING:
@@ -39,23 +38,22 @@ if TYPE_CHECKING:
     from starlette.requests import Request
     from starlette.responses import Response as StarletteResponse
 
-log: structlog.stdlib.BoundLogger = structlog.get_logger()
+log = get_logger(__name__)
 
 # Standard header name for correlation IDs (follows W3C Trace Context conventions)
 _CORRELATION_ID_HEADER: str = "X-Correlation-ID"
 
-# Key name for storing correlation ID in structlog context
+# Key name for storing correlation ID in request state
 CORRELATION_ID_CONTEXT_KEY: str = "correlation_id"
 
 
 class CorrelationIdMiddleware(BaseHTTPMiddleware):
-    """Extract or generate correlation ID and inject into structlog context.
+    """Extract or generate correlation ID and store on request state.
 
     For each request, this middleware:
     1. Checks for `X-Correlation-ID` header (trusted from frontend)
     2. Generates a new UUID4 if header not present
-    3. Stores in structlog.contextvars so all logs for this request include it
-    4. Makes available via request.state for error handlers
+    3. Stores on request.state for use by error handlers and log filters
 
     The correlation ID enables tracing a single user action or request flow
     across both frontend and backend systems using structured logs.
@@ -82,19 +80,12 @@ class CorrelationIdMiddleware(BaseHTTPMiddleware):
             str(uuid.uuid4()),
         )
 
-        # Store in structlog context so all logs for this request include it
-        structlog.contextvars.clear_contextvars()
-        structlog.contextvars.bind_contextvars(
-            **{CORRELATION_ID_CONTEXT_KEY: correlation_id}
-        )
-
-        # Also store on request.state for use by exception handlers
+        # Store on request.state for use by exception handlers
         request.state.correlation_id = correlation_id
 
         log.debug(
             "request_received",
-            method=request.method,
-            path=request.url.path,
+            extra={"method": request.method, "path": request.url.path},
         )
 
         response: StarletteResponse = await call_next(request)