Implement v1 URL shortener (Bearer API, public 302, landing, CI) (#2)

## Summary

- Standalone Django 6 shortener: Bearer `/api/links/` (create/list/detail/disable) and public `GET /<code>` 302
- Host split, target-host allowlist, named rotatable tokens; `short_url` from `PUBLIC_SHORT_URL`
- Landing page, DEBUG-only `/debug/` mint form, Django admin
- Docker/compose (host **8005**), Gitea CI like monica_site (PR tests, beta on merge, prod button)
- Caller contract in `API.md`

Closes #1. Infra follow-up: [server-infra#22](ai_ml_operations/server-infra#22).

## Test plan
- [ ] `cd site && uv run python manage.py test`
- [ ] `docker compose up --build` → http://127.0.0.1:8005/
- [ ] `POST /api/links/` with `Bearer monica:dev-only-token` → 201
- [ ] `GET /<code>` → 302 to allowlisted https URL
- [ ] No Bearer → 401; non-allowlisted host → 400
- [ ] `/debug/` only when `DEBUG=true`

Reviewed-on: #2

docker-compose.yml

@@ -0,0 +1,46 @@
+services:
+  db:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_DB: url_shortener
+      POSTGRES_USER: url_shortener
+      POSTGRES_PASSWORD: url_shortener
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U url_shortener -d url_shortener"]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+      start_period: 10s
+
+  web:
+    build: .
+    ports:
+      - "127.0.0.1:8005:8000"
+    volumes:
+      - ./site:/app/site
+    environment:
+      DJANGO_ENV: ${DJANGO_ENV:-dev}
+      DJANGO_SECRET_KEY: ${DJANGO_SECRET_KEY:-dev-only-change-me}
+      DJANGO_DEBUG: ${DJANGO_DEBUG:-true}
+      DJANGO_ALLOWED_HOSTS: ${DJANGO_ALLOWED_HOSTS:-localhost,127.0.0.1,0.0.0.0,web,url-shortener}
+      DATABASE_URL: ${COMPOSE_DATABASE_URL:-postgres://url_shortener:url_shortener@db:5432/url_shortener}
+      SHORT_DOMAIN: ${SHORT_DOMAIN:-localhost:8005}
+      PUBLIC_SHORT_URL: ${PUBLIC_SHORT_URL:-http://127.0.0.1:8005}
+      SHORT_PUBLIC_HOSTS: ${SHORT_PUBLIC_HOSTS:-go.mkdrealtor.com}
+      SHORT_API_HOSTS: ${SHORT_API_HOSTS:-localhost,127.0.0.1,0.0.0.0,web,url-shortener}
+      SHORTENER_API_TOKENS: ${SHORTENER_API_TOKENS:-monica:dev-only-token}
+      SHORT_ALLOWED_HOSTS: ${SHORT_ALLOWED_HOSTS:-mkdrealtor.com,aimloperations.com}
+      SHORT_CODE_LENGTH: ${SHORT_CODE_LENGTH:-6}
+      CLICK_IP_PEPPER: ${CLICK_IP_PEPPER:-dev-click-pepper-change-me}
+    networks:
+      default:
+        aliases:
+          - url-shortener
+    depends_on:
+      db:
+        condition: service_healthy
+
+volumes:
+  postgres_data: