feat: complete remaining Go backend modules

backend/internal/providers/mock.go

@@ -0,0 +1,18 @@
+package providers
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+)
+
+type Mock struct{ seed string }
+
+func NewMock(seed string) *Mock { return &Mock{seed: seed} }
+func (m *Mock) Submit(_ context.Context, r Request) (Result, error) {
+	sum := sha256.Sum256([]byte(m.seed + "\x00" + r.Capability + "\x00" + r.Prompt))
+	return Result{TaskID: "mock-" + hex.EncodeToString(sum[:8]), Status: StatusQueued}, nil
+}
+func (m *Mock) Query(_ context.Context, id string) (Result, error) {
+	return Result{TaskID: id, Status: StatusSucceeded, OutputURLs: []string{"/generated-results/" + id}}, nil
+}