LAN admin on 10.0.0.128 plus quick campaign mint form (#12)

Closes #11.

## Summary
- Serve Django admin on LAN IP `10.0.0.128` (not `piha.li`). Compose now publishes `0.0.0.0:8005` and passes `SHORT_ADMIN_HOSTS` into the container.
- Admin index has a campaign mint form: domain, campaign, source, metric. Save builds `https://{domain}/?utm_campaign=&utm_source=&utm_medium=` (metric) and shows a copyable short URL.
- Public `/admin/` on `piha.lc` / `piha.li` stays 404.

## Test plan
- [ ] `cd site && uv run python manage.py test`
- [ ] Recreate compose (`docker compose up --build`) so `WEB_BIND` / `SHORT_ADMIN_HOSTS` take effect
- [ ] From another LAN machine: `http://10.0.0.128:8005/admin/` (staff login) shows the mint form
- [ ] Save a link for an allowlisted domain, copy the short URL, confirm it 302s
- [ ] `https://piha.li/admin` still 404

Reviewed-on: #12

site/links/admin.py

@@ -2,8 +2,15 @@ from django.contrib import admin, messages
 from django.forms import ModelForm, ValidationError as FormValidationError
 from django.utils.html import format_html
 
+from links.forms import QuickMintForm, allowlisted_domain_choices
 from links.models import Click, ShortLink
-from links.services import ValidationError, generate_code, validate_target_url
+from links.services import (
+    CodeCollisionError,
+    ValidationError,
+    create_link,
+    generate_code,
+    validate_target_url,
+)
 
 admin.site.site_header = "URL shortener"
 admin.site.site_title = "Shortener admin"
@@ -156,3 +163,49 @@ class ClickAdmin(admin.ModelAdmin):
     def user_agent_short(self, obj: Click) -> str:
         ua = obj.user_agent or ""
         return (ua[:48] + "…") if len(ua) > 48 else (ua or "—")
+
+
+def _quick_mint_context(request, extra_context=None):
+    extra = extra_context.copy() if extra_context else {}
+    extra.setdefault("quick_mint_form", QuickMintForm())
+    extra.setdefault("created_link", None)
+    extra.setdefault("allowed_domains", allowlisted_domain_choices())
+    return extra
+
+
+def _mint_from_form(request, form: QuickMintForm):
+    token_name = request.user.get_username() if request.user.is_authenticated else "admin"
+    campaign = form.cleaned_data["campaign"].strip()
+    return create_link(
+        target_url=form.cleaned_data["target_url"],
+        title=campaign,
+        external_ref="",
+        expires_at=None,
+        token_name=token_name or "admin",
+    )
+
+
+_orig_index = admin.site.index
+
+
+def _admin_index(request, extra_context=None):
+    extra = _quick_mint_context(request, extra_context)
+    form = QuickMintForm(request.POST or None)
+    extra["quick_mint_form"] = form
+    if request.method == "POST":
+        if form.is_valid():
+            try:
+                link, minted = _mint_from_form(request, form)
+            except CodeCollisionError:
+                form.add_error(None, "Could not allocate a unique code.")
+            else:
+                extra["created_link"] = link
+                extra["quick_mint_form"] = QuickMintForm()
+                messages.success(
+                    request,
+                    "Short link created." if minted else "Existing active link returned.",
+                )
+    return _orig_index(request, extra)
+
+
+admin.site.index = _admin_index