feat: add gqlgen Recover that logs panics as one structured record (#206)

## Summary

gqlgen's `graphql.DefaultRecover` prints a recovered resolver panic and `debug.PrintStack()` to stderr, so the log collector stores every stack line as a separate, level-less entry (~60 entries per panic in production).

Adds `otelsetup.Recover` (`func(ctx context.Context, err any) error`, a `graphql.RecoverFunc` for gqlgen v0.17.95):

- logs the panic as **one** ERROR record via `slog.Default().ErrorContext` (the default logger services set up through `logging.SetupLogger`), with `panic`, `stack` (`debug.Stack()`) and `path` (GraphQL field path, when available) attributes
- records the error on the active span (`RecordError` with stack trace, `SetStatus(codes.Error)`)
- returns the generic `internal system error` gqlerror to the client, without the panic text

Services install it with `srv.SetRecoverFunc(otelsetup.Recover)`.

## Test plan

- `TestRecover`: error and non-error panic values; asserts the generic gqlerror, no panic text leak, exactly one ERROR log record with panic/stack attributes, and span status Error with an `exception` event carrying message and stacktrace (in-memory `tracetest.SpanRecorder`)
- `go test -race ./...` passes, `prek run --all-files` passes

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

https://claude.ai/code/session_01H6eVuR4Yt3m7TbDTDwyZJi
Reviewed-on: #206

recover.go

@@ -0,0 +1,41 @@
+package otelsetup
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"runtime/debug"
+
+	"github.com/99designs/gqlgen/graphql"
+	"github.com/vektah/gqlparser/v2/gqlerror"
+	"go.opentelemetry.io/otel/codes"
+	"go.opentelemetry.io/otel/trace"
+)
+
+// Recover is a gqlgen recover function, installed with srv.SetRecoverFunc(otelsetup.Recover).
+// Unlike graphql.DefaultRecover, which prints the stack to stderr line by line, it logs the
+// panic as a single structured ERROR record (via slog.Default) with the stack as an attribute,
+// records it on the active span and marks the span as failed. The client gets the same generic
+// "internal system error" as with the default, without the panic text.
+func Recover(ctx context.Context, err any) error {
+	stack := string(debug.Stack())
+	panicErr, ok := err.(error)
+	if !ok {
+		panicErr = fmt.Errorf("%v", err)
+	}
+
+	attrs := []any{
+		slog.String("panic", panicErr.Error()),
+		slog.String("stack", stack),
+	}
+	if fc := graphql.GetFieldContext(ctx); fc != nil {
+		attrs = append(attrs, slog.String("path", fc.Path().String()))
+	}
+	slog.Default().ErrorContext(ctx, "recovered panic in graphql resolver", attrs...)
+
+	span := trace.SpanFromContext(ctx)
+	span.RecordError(panicErr, trace.WithStackTrace(true))
+	span.SetStatus(codes.Error, "panic: "+panicErr.Error())
+
+	return gqlerror.Errorf("internal system error")
+}