feat: add /health endpoint and switch to fastmcp 2.0

- Switch from mcp package to fastmcp>=2.0 for Streamable HTTP support
  - Fixes 405 Method Not Allowed for MCP clients like Gemini CLI
  - MCP endpoint now at /mcp (POST) instead of /sse (GET)
  - Removes MCP_ALLOWED_HOSTS (handled at reverse proxy level)

- Add /health endpoint for Docker health checks
  - Returns {"status": "ok"} (200) or {"status": "degraded"} (503)
  - Enables health checks without transport security issues

- Add curl to Docker image for health checks
- Add healthcheck config to docker-compose files
- Add test-health and test-mcp Makefile targets
- Update documentation

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build dev logs stop test-sse clean rebuild
+.PHONY: build dev logs stop test-mcp test-health clean rebuild
 
 # Build the Docker image locally
 build:
@@ -18,15 +18,22 @@ 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 MCP endpoint (POST to /mcp)
+test-mcp:
+	@echo "Testing MCP endpoint..."
+	@curl -s -X POST http://localhost:8001/mcp \
+		-H "Content-Type: application/json" \
+		-d '{"jsonrpc": "2.0", "method": "initialize", "params": {"protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": {"name": "test", "version": "1.0"}}, "id": 1}' | head -200
 
 # Test root endpoint
 test-root:
 	@curl -s http://localhost:8001/ | head -20
 
+# Test health endpoint (bypasses MCP_ALLOWED_HOSTS)
+test-health:
+	@echo "Testing health endpoint..."
+	@curl -s http://localhost:8001/health
+
 # Full rebuild (no cache)
 rebuild:
 	docker compose -f docker-compose.dev.yml build --no-cache