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>

fly.toml

@@ -0,0 +1,34 @@
+# Fly.io configuration for the ExploreDNS web server.
+# Deploy with `make deploy` (or `flyctl deploy --remote-only`).
+# First-time setup: see "Deploying to Fly.io" in README.md.
+
+app = "exploredns"
+primary_region = "syd"
+
+[build]
+  dockerfile = "Dockerfile.web"
+
+[env]
+  # Public-exposure guards (web/api reads these at startup).
+  EXPLOREDNS_JOB_TIMEOUT = "5m"
+  EXPLOREDNS_MAX_JOBS = "8"
+
+[http_service]
+  internal_port = 8080
+  force_https = true
+  # Scale to zero when idle; SSE streams count as active connections,
+  # so machines are not stopped mid-traversal.
+  auto_stop_machines = "stop"
+  auto_start_machines = true
+  min_machines_running = 0
+
+  [[http_service.checks]]
+    interval = "30s"
+    timeout = "5s"
+    grace_period = "10s"
+    method = "GET"
+    path = "/api/health"
+
+[[vm]]
+  size = "shared-cpu-1x"
+  memory = "256mb"