feat(vars): project shared variables — ${{shared.KEY}} refs, CRUD, UI

- project_variables table (secret entries encrypted at rest, masked in API)
- resolveServiceEnv resolves shared.* before service lookup
- GET|PUT /projects/{id}/variables; CLI 'variables shared list|set|unset';
  MCP containr_shared_variables_{list,set}
- Settings tab → Shared Variables editor; env help text mentions shared refs

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/runtime.go

@@ -112,6 +112,9 @@ func resolveServiceEnv(db *database.DB, service Service) (map[string]string, err
 	}
 
 	resolve := func(serviceName, key string) (string, bool) {
+		if strings.EqualFold(serviceName, "shared") {
+			return resolveSharedVar(db, service.ProjectID, key)
+		}
 		id, ok := byName[strings.ToLower(serviceName)]
 		if !ok {
 			return "", false