build: add Fly.io deployment process

fly.toml (Dockerfile.web, scale-to-zero in syd, /api/health checks),
make deploy / deploy-status targets, a Gitea workflow deploying on v*
tags or manual dispatch via FLY_API_TOKEN, and README instructions
covering first-time setup and the server's runtime env knobs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Makefile

@@ -4,7 +4,7 @@ BUILD_DIR=bin
 GO=go
 GOFLAGS=-v
 
-.PHONY: build build-server build-all test lint clean cover
+.PHONY: build build-server build-all test lint clean cover deploy deploy-status
 
 build:
 	$(GO) build $(GOFLAGS) -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/exploredns
@@ -27,3 +27,12 @@ lint:
 clean:
 	rm -rf $(BUILD_DIR)
 	rm -f coverage.out coverage.html
+
+# Deploy the web server to Fly.io (requires flyctl and a configured app;
+# see "Deploying to Fly.io" in README.md).
+deploy:
+	flyctl deploy --remote-only
+
+deploy-status:
+	flyctl status
+	flyctl checks list