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/routers/projects.py

@@ -18,6 +18,7 @@ from ..schemas import (
     ProjectDetail,
     ProjectPrompts,
     ProjectPromptsUpdate,
+    ProjectSettingsUpdate,
     ProjectStats,
     ProjectSummary,
 )
@@ -63,13 +64,23 @@ def _get_registry_functions():
         sys.path.insert(0, str(root))
 
     from registry import (
+        get_project_concurrency,
         get_project_path,
         list_registered_projects,
         register_project,
+        set_project_concurrency,
         unregister_project,
         validate_project_path,
     )
-    return register_project, unregister_project, get_project_path, list_registered_projects, validate_project_path
+    return (
+        register_project,
+        unregister_project,
+        get_project_path,
+        list_registered_projects,
+        validate_project_path,
+        get_project_concurrency,
+        set_project_concurrency,
+    )
 
 
 router = APIRouter(prefix="/api/projects", tags=["projects"])
@@ -102,7 +113,8 @@ def get_project_stats(project_dir: Path) -> ProjectStats:
 async def list_projects():
     """List all registered projects."""
     _init_imports()
-    _, _, _, list_registered_projects, validate_project_path = _get_registry_functions()
+    (_, _, _, list_registered_projects, validate_project_path,
+     get_project_concurrency, _) = _get_registry_functions()
 
     projects = list_registered_projects()
     result = []
@@ -123,6 +135,7 @@ async def list_projects():
             path=info["path"],
             has_spec=has_spec,
             stats=stats,
+            default_concurrency=info.get("default_concurrency", 3),
         ))
 
     return result
@@ -132,7 +145,8 @@ async def list_projects():
 async def create_project(project: ProjectCreate):
     """Create a new project at the specified path."""
     _init_imports()
-    register_project, _, get_project_path, list_registered_projects, _ = _get_registry_functions()
+    (register_project, _, get_project_path, list_registered_projects,
+     _, _, _) = _get_registry_functions()
 
     name = validate_project_name(project.name)
     project_path = Path(project.path).resolve()
@@ -203,6 +217,7 @@ async def create_project(project: ProjectCreate):
         path=project_path.as_posix(),
         has_spec=False,  # Just created, no spec yet
         stats=ProjectStats(passing=0, total=0, percentage=0.0),
+        default_concurrency=3,
     )
 
 
@@ -210,7 +225,7 @@ async def create_project(project: ProjectCreate):
 async def get_project(name: str):
     """Get detailed information about a project."""
     _init_imports()
-    _, _, get_project_path, _, _ = _get_registry_functions()
+    (_, _, get_project_path, _, _, get_project_concurrency, _) = _get_registry_functions()
 
     name = validate_project_name(name)
     project_dir = get_project_path(name)
@@ -231,6 +246,7 @@ async def get_project(name: str):
         has_spec=has_spec,
         stats=stats,
         prompts_dir=str(prompts_dir),
+        default_concurrency=get_project_concurrency(name),
     )
 
 
@@ -244,7 +260,7 @@ async def delete_project(name: str, delete_files: bool = False):
         delete_files: If True, also delete the project directory and files
     """
     _init_imports()
-    _, unregister_project, get_project_path, _, _ = _get_registry_functions()
+    (_, unregister_project, get_project_path, _, _, _, _) = _get_registry_functions()
 
     name = validate_project_name(name)
     project_dir = get_project_path(name)
@@ -280,7 +296,7 @@ async def delete_project(name: str, delete_files: bool = False):
 async def get_project_prompts(name: str):
     """Get the content of project prompt files."""
     _init_imports()
-    _, _, get_project_path, _, _ = _get_registry_functions()
+    (_, _, get_project_path, _, _, _, _) = _get_registry_functions()
 
     name = validate_project_name(name)
     project_dir = get_project_path(name)
@@ -313,7 +329,7 @@ async def get_project_prompts(name: str):
 async def update_project_prompts(name: str, prompts: ProjectPromptsUpdate):
     """Update project prompt files."""
     _init_imports()
-    _, _, get_project_path, _, _ = _get_registry_functions()
+    (_, _, get_project_path, _, _, _, _) = _get_registry_functions()
 
     name = validate_project_name(name)
     project_dir = get_project_path(name)
@@ -343,7 +359,7 @@ async def update_project_prompts(name: str, prompts: ProjectPromptsUpdate):
 async def get_project_stats_endpoint(name: str):
     """Get current progress statistics for a project."""
     _init_imports()
-    _, _, get_project_path, _, _ = _get_registry_functions()
+    (_, _, get_project_path, _, _, _, _) = _get_registry_functions()
 
     name = validate_project_name(name)
     project_dir = get_project_path(name)
@@ -355,3 +371,40 @@ async def get_project_stats_endpoint(name: str):
         raise HTTPException(status_code=404, detail="Project directory not found")
 
     return get_project_stats(project_dir)
+
+
+@router.patch("/{name}/settings", response_model=ProjectDetail)
+async def update_project_settings(name: str, settings: ProjectSettingsUpdate):
+    """Update project-level settings (concurrency, etc.)."""
+    _init_imports()
+    (_, _, get_project_path, _, _, get_project_concurrency,
+     set_project_concurrency) = _get_registry_functions()
+
+    name = validate_project_name(name)
+    project_dir = get_project_path(name)
+
+    if not project_dir:
+        raise HTTPException(status_code=404, detail=f"Project '{name}' not found")
+
+    if not project_dir.exists():
+        raise HTTPException(status_code=404, detail="Project directory not found")
+
+    # Update concurrency if provided
+    if settings.default_concurrency is not None:
+        success = set_project_concurrency(name, settings.default_concurrency)
+        if not success:
+            raise HTTPException(status_code=500, detail="Failed to update concurrency")
+
+    # Return updated project details
+    has_spec = _check_spec_exists(project_dir)
+    stats = get_project_stats(project_dir)
+    prompts_dir = _get_project_prompts_dir(project_dir)
+
+    return ProjectDetail(
+        name=name,
+        path=project_dir.as_posix(),
+        has_spec=has_spec,
+        stats=stats,
+        prompts_dir=str(prompts_dir),
+        default_concurrency=get_project_concurrency(name),
+    )