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

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

app/frontend/src/features/service/pages/ServiceDetailPage.tsx

@@ -1219,8 +1219,9 @@ export function ServiceDetailPage() {
                   <h2 className="text-lg font-semibold text-[var(--text-primary)]">Environment Variables</h2>
                   <p className="text-sm text-[var(--text-secondary)]">
                     Runtime values injected at deploy. Reference another service's variable with{' '}
-                    <code className="mono text-[var(--accent-primary)]">{'${{service.KEY}}'}</code> — e.g.{' '}
-                    <code className="mono text-[var(--accent-primary)]">{'${{postgres.POSTGRES_PASSWORD}}'}</code>
+                    <code className="mono text-[var(--accent-primary)]">{'${{service.KEY}}'}</code> or a project
+                    shared variable with{' '}
+                    <code className="mono text-[var(--accent-primary)]">{'${{shared.KEY}}'}</code>
                   </p>
                 </div>
               </div>