Phase 3c: producer SDK (internal/client)

[StrangeNoob]

Summary

Third Phase 3 sub-project: a small, self-contained Go HTTP SDK for the Relay API so producers can drive the queue without touching Redis. cmd/demo is refactored to consume it — proving the SDK end-to-end and establishing the real producer topology (producer → HTTP API → broker). No new Go dependency (stdlib only); the atomic claim and at-least-once delivery are untouched.

internal/client (new, stdlib only)

• New(baseURL, ...Option) with WithHTTPClient/WithTimeout (default ~10s).
• Enqueue(ctx, queue, payload, ...EnqueueOption) — WithDelay/WithPriority/WithIdempotencyKey; 409 → ErrDuplicate. Priority is a *int so WithPriority(0) sends priority:0 while an unset priority is omitted (mirrors the API).
• Stats, ListDLQ, Requeue (404 → ErrNotFound), Queues.
• Typed errors: sentinels ErrDuplicate/ErrNotFound; *APIError{Status, Message} from the {"error":…} envelope for other non-2xx. A shared do helper handles marshalling, status mapping, and %w wrapping.
• No broker/job/Redis import — a producer importing the SDK doesn't drag in the queue internals.

cmd/demo (refactor)

Now a pure SDK consumer: -server http://localhost:8080 replaces -redis; it client.New(...) + Enqueues, treating ErrDuplicate as benign. (It now needs cmd/server running.)

Design & plan

• Spec: docs/superpowers/specs/2026-06-09-relay-phase3c-producer-sdk-design.md
• Plan: docs/superpowers/plans/2026-06-09-relay-phase3c-producer-sdk.md

Test plan

• go build ./..., go vet ./..., gofmt -l internal/ cmd/ clean
• go test -race ./... — all packages pass against a real Redis (client tests on DB 11; broker 15 / worker 14 / metrics 13 / api 12 / client 11 — parallel-safe)
• Hermetic httptest unit tests: request method/path/body, response decode, and error mapping (409→ErrDuplicate, 404→ErrNotFound, 500→APIError); priority-0-sent vs unset-omitted
• Wire-compat round-trip: client → api.New(real broker) → Redis proves the client DTOs match the live server JSON (no field drift)
• internal/client confirmed stdlib-only (go list -deps shows no broker/job/go-redis); go.mod unchanged
• cmd/demo smoke-tested end-to-end against a running server

[StrangeNoob]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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