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>

.gitea/workflows/deploy.yml

@@ -0,0 +1,27 @@
+name: Deploy
+
+# Deploys the web server to Fly.io.
+# Triggers: pushing a version tag (v*), or manual dispatch.
+# Requires the FLY_API_TOKEN repository secret (create with
+# `flyctl tokens create deploy -x 999999h`).
+
+on:
+  push:
+    tags: ["v*"]
+  workflow_dispatch:
+
+jobs:
+  deploy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install flyctl
+        run: |
+          curl -L https://fly.io/install.sh | sh
+          echo "$HOME/.fly/bin" >> "$GITHUB_PATH"
+
+      - name: Deploy
+        run: flyctl deploy --remote-only
+        env:
+          FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN }}