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/storage/storage.go

@@ -24,7 +24,7 @@ var (
 //
 // Keys are slash-delimited paths relative to the storage root:
 //
-//	files/{channel_id}/{file_id}_{filename}
+//	ext/{package_id}/{file_id}_{filename}
 //
 // Implementations must create intermediate directories as needed.
 type ObjectStore interface {
@@ -43,8 +43,8 @@ type ObjectStore interface {
 	Delete(ctx context.Context, key string) error
 
 	// DeletePrefix removes all objects under the given prefix.
-	// Used for channel deletion (bulk cleanup).
-	// Example: DeletePrefix(ctx, "files/{channel_id}/")
+	// Used for bulk cleanup (e.g. extension uninstall).
+	// Example: DeletePrefix(ctx, "ext/{package_id}/")
 	DeletePrefix(ctx context.Context, prefix string) error
 
 	// Exists checks if an object exists at key without reading it.