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/webhook/webhook.go

@@ -29,7 +29,6 @@ type Payload struct {
 	RunID       string    `json:"run_id,omitempty"`
 	TaskName    string    `json:"task_name,omitempty"`
 	WorkflowID  string    `json:"workflow_id,omitempty"`
-	ChannelID   string    `json:"channel_id,omitempty"`
 	Status      string    `json:"status"`
 	CompletedAt time.Time `json:"completed_at"`
 	Output      string    `json:"output,omitempty"`       // last assistant message or relay payload