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/lib/api.ts

@@ -6,6 +6,7 @@ import type {
   ProjectSummary,
   ProjectDetail,
   ProjectPrompts,
+  ProjectSettingsUpdate,
   FeatureListResponse,
   Feature,
   FeatureCreate,
@@ -100,6 +101,16 @@ export async function updateProjectPrompts(
   })
 }
 
+export async function updateProjectSettings(
+  name: string,
+  settings: ProjectSettingsUpdate
+): Promise<ProjectDetail> {
+  return fetchJSON(`/projects/${encodeURIComponent(name)}/settings`, {
+    method: 'PATCH',
+    body: JSON.stringify(settings),
+  })
+}
+
 // ============================================================================
 // Features API
 // ============================================================================

ui/src/lib/types.ts

@@ -15,6 +15,7 @@ export interface ProjectSummary {
   path: string
   has_spec: boolean
   stats: ProjectStats
+  default_concurrency: number
 }
 
 export interface ProjectDetail extends ProjectSummary {
@@ -536,6 +537,10 @@ export interface SettingsUpdate {
   testing_agent_ratio?: number
 }
 
+export interface ProjectSettingsUpdate {
+  default_concurrency?: number
+}
+
 // ============================================================================
 // Schedule Types
 // ============================================================================