fix(backend): correct generic type parameter in withHeaderCarrier helper

Problem:
The withHeaderCarrier generic function had a type mismatch that caused compilation
errors in CI. The function used `T proto.Message` constraint, but Connect's Response
type expects the non-pointer message type while protobuf methods return pointers.

Error from CI:
  type T of resp does not match *T (cannot infer T)

This occurred because:
- Connect methods expect: *connect.Response[v1pb.CreateSessionResponse]
- Service methods return: (*v1pb.CreateSessionResponse, error)
- Old signature: fn func(context.Context) (T, error) with T proto.Message
- This caused T to be inferred as *v1pb.CreateSessionResponse
- Leading to return type: *connect.Response[*v1pb.CreateSessionResponse] (wrong!)

Solution:
Changed generic signature to explicitly handle the pointer/non-pointer distinction:
- New signature: fn func(context.Context) (*T, error) with T any
- T is now the non-pointer type (e.g., v1pb.CreateSessionResponse)
- fn returns *T (e.g., *v1pb.CreateSessionResponse)
- Return type is correctly: *connect.Response[T] (e.g., *connect.Response[v1pb.CreateSessionResponse])

Also removed unused "google.golang.org/protobuf/proto" import and improved documentation
to clarify the T vs *T distinction.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: boojack

server/router/api/v1/header_carrier.go

@@ -6,7 +6,6 @@ import (
 	"connectrpc.com/connect"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/metadata"
-	"google.golang.org/protobuf/proto"
 )
 
 // headerCarrierKey is the context key for storing headers to be set in the response.
@@ -91,14 +90,17 @@ func SetResponseHeader(ctx context.Context, key, value string) error {
 // It injects a HeaderCarrier into the context, calls the service method,
 // and applies any headers from the carrier to the Connect response.
 //
+// The generic parameter T is the non-pointer protobuf message type (e.g., v1pb.CreateSessionResponse),
+// while fn returns *T (the pointer type) as is standard for protobuf messages.
+//
 // Usage in Connect wrappers:
 //
-//	func (s *ConnectServiceHandler) CreateSession(ctx context.Context, req *connect.Request[...]) (*connect.Response[...], error) {
+//	func (s *ConnectServiceHandler) CreateSession(ctx context.Context, req *connect.Request[v1pb.CreateSessionRequest]) (*connect.Response[v1pb.CreateSessionResponse], error) {
 //	    return withHeaderCarrier(ctx, func(ctx context.Context) (*v1pb.CreateSessionResponse, error) {
 //	        return s.APIV1Service.CreateSession(ctx, req.Msg)
 //	    })
 //	}
-func withHeaderCarrier[T proto.Message](ctx context.Context, fn func(context.Context) (T, error)) (*connect.Response[T], error) {
+func withHeaderCarrier[T any](ctx context.Context, fn func(context.Context) (*T, error)) (*connect.Response[T], error) {
 	// Inject header carrier for Connect protocol
 	ctx = WithHeaderCarrier(ctx)