feat: persist concurrent agents slider at project level

Add `default_concurrency` column to the projects table in the registry
database, allowing each project to remember its preferred concurrency
setting (1-5 agents). The value persists across page refreshes and
app restarts.

Backend changes:
- Add `default_concurrency` column to Project model in registry.py
- Add database migration for existing databases (ALTER TABLE)
- Add get/set_project_concurrency() CRUD functions
- Add ProjectSettingsUpdate schema with validation
- Add PATCH /{name}/settings endpoint in projects router
- Include default_concurrency in ProjectSummary/ProjectDetail responses

Frontend changes:
- Add default_concurrency to ProjectSummary TypeScript interface
- Add ProjectSettingsUpdate type and updateProjectSettings API function
- Add useUpdateProjectSettings React Query mutation hook
- Update AgentControl to accept defaultConcurrency prop
- Sync local state when project changes via useEffect
- Debounce slider changes (500ms) before saving to backend
- Pass defaultConcurrency from selectedProjectData in App.tsx

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/hooks/useProjects.ts

@@ -4,7 +4,7 @@
 
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import * as api from '../lib/api'
-import type { FeatureCreate, FeatureUpdate, ModelsResponse, Settings, SettingsUpdate } from '../lib/types'
+import type { FeatureCreate, FeatureUpdate, ModelsResponse, ProjectSettingsUpdate, Settings, SettingsUpdate } from '../lib/types'
 
 // ============================================================================
 // Projects
@@ -48,6 +48,19 @@ export function useDeleteProject() {
   })
 }
 
+export function useUpdateProjectSettings(projectName: string) {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: (settings: ProjectSettingsUpdate) =>
+      api.updateProjectSettings(projectName, settings),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['projects'] })
+      queryClient.invalidateQueries({ queryKey: ['project', projectName] })
+    },
+  })
+}
+
 // ============================================================================
 // Features
 // ============================================================================