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>

server/schemas.py

@@ -45,6 +45,7 @@ class ProjectSummary(BaseModel):
     path: str
     has_spec: bool
     stats: ProjectStats
+    default_concurrency: int = 3
 
 
 class ProjectDetail(BaseModel):
@@ -54,6 +55,7 @@ class ProjectDetail(BaseModel):
     has_spec: bool
     stats: ProjectStats
     prompts_dir: str
+    default_concurrency: int = 3
 
 
 class ProjectPrompts(BaseModel):
@@ -70,6 +72,18 @@ class ProjectPromptsUpdate(BaseModel):
     coding_prompt: str | None = None
 
 
+class ProjectSettingsUpdate(BaseModel):
+    """Request schema for updating project-level settings."""
+    default_concurrency: int | None = None
+
+    @field_validator('default_concurrency')
+    @classmethod
+    def validate_concurrency(cls, v: int | None) -> int | None:
+        if v is not None and (v < 1 or v > 5):
+            raise ValueError("default_concurrency must be between 1 and 5")
+        return v
+
+
 # ============================================================================
 # Feature Schemas
 # ============================================================================