feat: Docker images, compose stacks, nginx config, Gitea CI

- site image: multi-stage node build -> unprivileged nginx (non-root, read-only)
- nginx: CSP + security headers, immutable asset caching, revalidated HTML,
  canonical trailing slash, preserved /Linkedin 301, legacy-WP 410s, custom 404
- externalise theme-init so CSP uses script-src 'self' (no inline hash)
- prod + dev compose; .env.example; relay Dockerfile fixed (image ships app user)
- Gitea Actions: quality, e2e, lighthouse budgets, relay build, image push on main
- verified: both images build; relay healthz 200; site serves EN/NO with CSP + redirect

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

deploy/docker-compose.dev.yml

@@ -0,0 +1,32 @@
+# Development stack (DOCKER_SPEC §1). Contributors need only Docker — Astro HMR and
+# the relay with dotnet watch, with /api/contact proxied by the Astro dev server.
+
+services:
+  site-dev:
+    image: node:22-alpine
+    working_dir: /app
+    command: sh -c "corepack enable && pnpm install && pnpm dev --host --port 4321"
+    environment:
+      - ASTRO_TELEMETRY_DISABLED=1
+    volumes:
+      - ../site:/app
+      - site_node_modules:/app/node_modules
+    ports:
+      - '4321:4321'
+
+  relay-dev:
+    image: mcr.microsoft.com/dotnet/sdk:9.0-alpine
+    working_dir: /src
+    command: sh -c "dotnet watch run --urls http://+:8081 --non-interactive"
+    environment:
+      - DOTNET_USE_POLLING_FILE_WATCHER=1
+      - Smtp__Host=${SMTP_HOST:-}
+      - Relay__ToAddress=${RELAY_TO:-dev@example.com}
+      - Relay__AllowedOrigin=http://localhost:4321
+    volumes:
+      - ../relay:/src
+    ports:
+      - '8081:8081'
+
+volumes:
+  site_node_modules: