Feat v0.7.6 code hygiene + test coverage

Clean up channels-era dead code, align migrations, add 92 tests,
and fix backup download + admin storage tab bugs before v0.8.x
kernel expansion.

Critical fixes:
- Remove `channels` from allowedViews in db_module.go
- Fix 5 dead API routes in workflow.html → public workflow API
- Fix RenderWorkflow handler to use route param as entry token

Dead code removal:
- Delete SeedTestChannel(), RunContext.ChannelID, webhook.ChannelID
- Fix stale comments in storage.go, prometheus.go, workflow_module.go

Migration hygiene:
- Add SQLite placeholder 013_cluster_registry.sql, renumber to 014
- Compat rename in migrate.go for existing SQLite databases
- Document missing migration 008 in both 009 files

Test coverage:
- 82 workflow routing tests (all operators, branch rules, stage resolution)
- 10 middleware tests (permissions, admin, rate limiter)

Bug fixes:
- Remove dead Admin Storage tab from System category
- Fix backup download: sw.auth.token() → sw.auth._getToken()

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/database/testhelper.go

@@ -427,25 +427,6 @@ func SeedAdminsGroupMember(t *testing.T, userID string) {
 	}
 }
 
-// SeedTestChannel creates a test channel owned by userID and returns the channel ID.
-func SeedTestChannel(t *testing.T, userID, title string) string {
-	t.Helper()
-	if IsSQLite() {
-		id := uuid.New().String()
-		_, err := DB.Exec(`INSERT INTO channels (id, user_id, title) VALUES (?, ?, ?)`, id, userID, title)
-		if err != nil {
-			t.Fatalf("SeedTestChannel: %v", err)
-		}
-		return id
-	}
-	var id string
-	err := DB.QueryRow(`INSERT INTO channels (user_id, title) VALUES ($1, $2) RETURNING id`, userID, title).Scan(&id)
-	if err != nil {
-		t.Fatalf("SeedTestChannel: %v", err)
-	}
-	return id
-}
-
 // SeedTestTeam creates a test team and returns the team ID.
 func SeedTestTeam(t *testing.T, name, createdBy string) string {
 	t.Helper()