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

CLAUDE.md

@@ -28,6 +28,12 @@ defer shutdown(ctx)
 
 // Wrap HTTP handlers with tracing
 http.Handle("/", otelsetup.Handler(myHandler))
+
+// gqlgen server hooks
+srv.AroundOperations(otelsetup.AroundOperations)
+srv.AroundRootFields(otelsetup.AroundRootFields)
+srv.AroundFields(otelsetup.AroundFields)
+srv.SetRecoverFunc(otelsetup.Recover)
 ```
 
 ### Features
@@ -37,6 +43,8 @@ http.Handle("/", otelsetup.Handler(myHandler))
 - **Logging** - Stdout log exporter
 - **Propagation** - TraceContext and Baggage propagation
 - **HTTP Handler** - Middleware for automatic span creation
+- **gqlgen hooks** - Spans per operation, root field and resolver field
+- **gqlgen recover** - `Recover` logs a resolver panic as one ERROR record via `slog.Default()` (attributes `panic`, `stack`, `path`), records it on the active span with status Error, and returns the generic "internal system error" to the client (gqlgen's default prints the stack to stderr line by line)
 
 ### Environment Variables