feat: add /health endpoint bypassing transport security

- Migrate from SSE to HTTP transport using fastmcp>=2.0
- Add /health endpoint for Docker health checks and load balancers
- Remove MCP_ALLOWED_HOSTS (no longer needed with http_app approach)
- Add lifespan handler for proper task group initialization
- Install curl in Docker image for health checks
- Update Makefile with test-health and test-mcp targets
- Update documentation to reflect new endpoint structure

Fixes: Health check fails with 421 Misdirected Request when
MCP_ALLOWED_HOSTS doesn't include localhost

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build dev logs stop test-sse clean
+.PHONY: build dev logs stop test-health clean
 
 # Build the Docker image locally
 build:
@@ -18,14 +18,15 @@ logs:
 stop:
 	docker compose -f docker-compose.dev.yml down
 
-# Test SSE endpoint (5 second timeout)
-test-sse:
-	@echo "Testing SSE endpoint (5s timeout)..."
-	@curl -N --max-time 5 http://localhost:8001/sse 2>/dev/null || echo "\n[Timeout - this is expected for SSE]"
+# Test health endpoint
+test-health:
+	@echo "Testing health endpoint..."
+	@curl -s http://localhost:8001/health | jq .
 
-# Test root endpoint
-test-root:
-	@curl -s http://localhost:8001/ | head -20
+# Test MCP endpoint (should return 406 for GET)
+test-mcp:
+	@echo "Testing MCP endpoint..."
+	@curl -s -o /dev/null -w "%{http_code}" http://localhost:8001/mcp
 
 # Full rebuild (no cache)
 rebuild: