chore: cleanup sprint — dead CSS, dedup helpers, handler refactor

Remove ~126 lines of orphaned CSS from tile slim-down and old detail
layout. Consolidate 4 duplicate duration formatters into shared
elapsed()/fmtElapsed() helpers. Break 160-line Result handler into
focused sub-functions. Implement real Hub.Shutdown() (was a no-op).
Standardize agent error responses to JSON. Replace panic() in router
init with error return. Extract magic numbers as named constants.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/store/runs.go

@@ -10,6 +10,8 @@ import (
 	"vetting/internal/model"
 )
 
+const defaultRunListLimit = 20
+
 type Runs struct {
 	DB *sql.DB
 }
@@ -182,7 +184,7 @@ func (r *Runs) LatestForHost(ctx context.Context, hostID int64) (*model.Run, err
 // can't scan the whole history into memory.
 func (r *Runs) ListForHost(ctx context.Context, hostID int64, limit int) ([]model.Run, error) {
 	if limit <= 0 {
-		limit = 20
+		limit = defaultRunListLimit
 	}
 	rows, err := r.DB.QueryContext(ctx, `
 		SELECT id, host_id, state, COALESCE(result,''), COALESCE(failed_stage,''),