feat(dag_engine): json tags lowercase + gitlink dag_engine_ui HTTP layer (issue 0095 step 3)

- store/store.go: anade tags JSON lowercase a DagRun + DagStepResult para que REST y WS devuelvan misma forma.
- cpp/apps/dag_engine_ui: gitlink al SHA con http_client + data_http.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/dag_engine/store/store.go

@@ -46,14 +46,14 @@ func (db *DB) Conn() *sql.DB {
 
 // DagRun mirrors infra.DagRun for the store layer.
 type DagRun struct {
-	ID         string
+	ID         string     `json:"id"`
-	DagName    string
+	DagName    string     `json:"dag_name"`
-	DagPath    string
+	DagPath    string     `json:"dag_path"`
-	Status     string
+	Status     string     `json:"status"`
-	Trigger    string
+	Trigger    string     `json:"trigger"`
-	StartedAt  time.Time
+	StartedAt  time.Time  `json:"started_at"`
-	FinishedAt *time.Time
+	FinishedAt *time.Time `json:"finished_at,omitempty"`
-	Error      string
+	Error      string     `json:"error,omitempty"`
 }
 
 // CreateRun inserts a new run record.
@@ -129,17 +129,17 @@ func (db *DB) ListRuns(dagName string, limit, offset int) ([]DagRun, int, error)
 
 // DagStepResult mirrors infra.DagStepResult for the store layer.
 type DagStepResult struct {
-	ID         string
+	ID         string     `json:"id"`
-	RunID      string
+	RunID      string     `json:"run_id"`
-	StepName   string
+	StepName   string     `json:"step_name"`
-	Status     string
+	Status     string     `json:"status"`
-	ExitCode   int
+	ExitCode   int        `json:"exit_code"`
-	Stdout     string
+	Stdout     string     `json:"stdout,omitempty"`
-	Stderr     string
+	Stderr     string     `json:"stderr,omitempty"`
-	StartedAt  *time.Time
+	StartedAt  *time.Time `json:"started_at,omitempty"`
-	FinishedAt *time.Time
+	FinishedAt *time.Time `json:"finished_at,omitempty"`
-	DurationMs int64
+	DurationMs int64      `json:"duration_ms"`
-	Error      string
+	Error      string     `json:"error,omitempty"`
 }
 
 // InsertStepResult inserts a new step result.