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

.env.prod.example

@@ -0,0 +1,54 @@
+# Secret env files for server-infra deploy.
+# Copy to the control node (never commit):
+#   ~/Documents/secrets/url_shortening_service/url_shortening_service_prod.env
+#   ~/Documents/secrets/url_shortening_service/url_shortening_service_beta.env
+#
+# Docker Compose: if a secret contains $ (e.g. in DATABASE_URL password), escape each
+# $ as $$ or compose will treat $word as a variable.
+
+# =============================================================================
+# PROD
+# =============================================================================
+DJANGO_ENV=prod
+DJANGO_DEBUG=false
+DJANGO_SECRET_KEY=replace-with-a-long-random-secret
+# Public short host AND public API hostname (plus docker names if used).
+DJANGO_ALLOWED_HOSTS=go.mkdrealtor.com,shortener.aimloperations.com,url-shortener,web
+
+# Shared external Postgres
+DATABASE_URL=postgres://westfarn:replace-db-password@10.0.0.230:5432/url_shortener
+
+# Host port (must match server-infra). NPM proxies SHORT_DOMAIN here.
+WEB_PORT=8005
+
+SHORT_DOMAIN=go.mkdrealtor.com
+PUBLIC_SHORT_URL=https://go.mkdrealtor.com
+SHORT_PUBLIC_HOSTS=go.mkdrealtor.com
+SHORT_API_HOSTS=shortener.aimloperations.com,url-shortener,web
+SHORT_ADMIN_HOSTS=localhost,127.0.0.1
+# Generate: python -c "import secrets; print(secrets.token_urlsafe(32))"
+# This token is the only thing that authorizes minting. Treat it as a secret.
+# monica_site sends: Authorization: Bearer monica:<same-secret>
+SHORTENER_API_TOKENS=monica:replace-with-token-urlsafe-32
+SHORT_ALLOWED_HOSTS=mkdrealtor.com
+SHORT_CODE_LENGTH=6
+CLICK_IP_PEPPER=replace-with-a-distinct-pepper
+
+GUNICORN_WORKERS=2
+GUNICORN_BIND=0.0.0.0:8000
+
+# =============================================================================
+# BETA overrides
+# File: url_shortening_service_beta.env
+# =============================================================================
+# DJANGO_ENV=beta
+# DJANGO_DEBUG=false
+# DJANGO_SECRET_KEY=replace-with-a-different-beta-secret
+# DJANGO_ALLOWED_HOSTS=go-beta.example.com,shortener-beta.aimloperations.com,url-shortener,web
+# DATABASE_URL=postgres://westfarn:replace-db-password@10.0.0.230:5432/url_shortener_beta
+# WEB_PORT=8015
+# SHORT_DOMAIN=go-beta.example.com
+# PUBLIC_SHORT_URL=https://go-beta.example.com
+# SHORT_PUBLIC_HOSTS=go-beta.example.com
+# SHORTENER_API_TOKENS=monica:replace-with-a-different-token
+# CLICK_IP_PEPPER=replace-with-a-different-pepper