feat(canvas): Add file browser, introspection, and improve node flow

Phase 1-7 of Canvas V4 Ralph Loop implementation:

Backend:
- Add /api/files routes for browsing model files
- Add /api/nx routes for NX model introspection
- Add NXIntrospector service to discover expressions and extractors
- Add health check with database status

Frontend:
- Add FileBrowser component for selecting .sim/.prt/.fem files
- Add IntrospectionPanel to discover expressions and extractors
- Update NodeConfigPanel with browse and introspect buttons
- Update schema with NODE_HANDLES for proper flow direction
- Update validation for correct DesignVar -> Model -> Solver flow
- Update useCanvasStore.addNode() to accept custom data

Flow correction: Design Variables now connect TO Model (as source),
not FROM Model. This matches the actual data flow in optimization.

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

atomizer-dashboard/backend/api/main.py

@@ -13,7 +13,7 @@ import sys
 # Add parent directory to path to import optimization_engine
 sys.path.append(str(Path(__file__).parent.parent.parent.parent))
 
-from api.routes import optimization, claude, terminal, insights, context
+from api.routes import optimization, claude, terminal, insights, context, files, nx
 from api.websocket import optimization_stream
 
 
@@ -58,6 +58,8 @@ app.include_router(claude.router, prefix="/api/claude", tags=["claude"])
 app.include_router(terminal.router, prefix="/api/terminal", tags=["terminal"])
 app.include_router(insights.router, prefix="/api/insights", tags=["insights"])
 app.include_router(context.router, prefix="/api/context", tags=["context"])
+app.include_router(files.router, prefix="/api/files", tags=["files"])
+app.include_router(nx.router, prefix="/api/nx", tags=["nx"])
 
 @app.get("/")
 async def root():
@@ -67,8 +69,20 @@ async def root():
 
 @app.get("/health")
 async def health_check():
-    """Health check endpoint"""
-    return {"status": "healthy"}
+    """Health check endpoint with database status"""
+    try:
+        from api.services.conversation_store import ConversationStore
+        store = ConversationStore()
+        # Test database by creating/getting a health check session
+        store.get_session("health_check")
+        db_status = "connected"
+    except Exception as e:
+        db_status = f"error: {str(e)}"
+
+    return {
+        "status": "healthy" if db_status == "connected" else "degraded",
+        "database": db_status,
+    }
 
 if __name__ == "__main__":
     import uvicorn