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>

registry.py

@@ -16,7 +16,7 @@ from datetime import datetime
 from pathlib import Path
 from typing import Any
 
-from sqlalchemy import Column, DateTime, String, create_engine
+from sqlalchemy import Column, DateTime, Integer, String, create_engine, text
 from sqlalchemy.ext.declarative import declarative_base
 from sqlalchemy.orm import sessionmaker
 
@@ -85,6 +85,7 @@ class Project(Base):
     name = Column(String(50), primary_key=True, index=True)
     path = Column(String, nullable=False)  # POSIX format for cross-platform
     created_at = Column(DateTime, nullable=False)
+    default_concurrency = Column(Integer, nullable=False, default=3)
 
 
 class Settings(Base):
@@ -146,12 +147,26 @@ def _get_engine():
                     }
                 )
                 Base.metadata.create_all(bind=_engine)
+                _migrate_add_default_concurrency(_engine)
                 _SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=_engine)
                 logger.debug("Initialized registry database at: %s", db_path)
 
     return _engine, _SessionLocal
 
 
+def _migrate_add_default_concurrency(engine) -> None:
+    """Add default_concurrency column if missing (for existing databases)."""
+    with engine.connect() as conn:
+        result = conn.execute(text("PRAGMA table_info(projects)"))
+        columns = [row[1] for row in result.fetchall()]
+        if "default_concurrency" not in columns:
+            conn.execute(text(
+                "ALTER TABLE projects ADD COLUMN default_concurrency INTEGER DEFAULT 3"
+            ))
+            conn.commit()
+            logger.info("Migrated projects table: added default_concurrency column")
+
+
 @contextmanager
 def _get_session():
     """
@@ -307,7 +322,8 @@ def list_registered_projects() -> dict[str, dict[str, Any]]:
         return {
             p.name: {
                 "path": p.path,
-                "created_at": p.created_at.isoformat() if p.created_at else None
+                "created_at": p.created_at.isoformat() if p.created_at else None,
+                "default_concurrency": getattr(p, 'default_concurrency', 3) or 3
             }
             for p in projects
         }
@@ -333,7 +349,8 @@ def get_project_info(name: str) -> dict[str, Any] | None:
             return None
         return {
             "path": project.path,
-            "created_at": project.created_at.isoformat() if project.created_at else None
+            "created_at": project.created_at.isoformat() if project.created_at else None,
+            "default_concurrency": getattr(project, 'default_concurrency', 3) or 3
         }
     finally:
         session.close()
@@ -362,6 +379,55 @@ def update_project_path(name: str, new_path: Path) -> bool:
     return True
 
 
+def get_project_concurrency(name: str) -> int:
+    """
+    Get project's default concurrency (1-5).
+
+    Args:
+        name: The project name.
+
+    Returns:
+        The default concurrency value (defaults to 3 if not set or project not found).
+    """
+    _, SessionLocal = _get_engine()
+    session = SessionLocal()
+    try:
+        project = session.query(Project).filter(Project.name == name).first()
+        if project is None:
+            return 3
+        return getattr(project, 'default_concurrency', 3) or 3
+    finally:
+        session.close()
+
+
+def set_project_concurrency(name: str, concurrency: int) -> bool:
+    """
+    Set project's default concurrency (1-5).
+
+    Args:
+        name: The project name.
+        concurrency: The concurrency value (1-5).
+
+    Returns:
+        True if updated, False if project wasn't found.
+
+    Raises:
+        ValueError: If concurrency is not between 1 and 5.
+    """
+    if concurrency < 1 or concurrency > 5:
+        raise ValueError("concurrency must be between 1 and 5")
+
+    with _get_session() as session:
+        project = session.query(Project).filter(Project.name == name).first()
+        if not project:
+            return False
+
+        project.default_concurrency = concurrency
+
+    logger.info("Set project '%s' default_concurrency to %d", name, concurrency)
+    return True
+
+
 # =============================================================================
 # Validation Functions
 # =============================================================================