Extend the public API: news excerpts/detail, game team logos, sponsor logo dimensions, player licenses

- news: NewsItemOut gains `excerpt` (truncated body); GET /news/{slug}/
  fetches a single item. slug already auto-populates on save, but a
  data migration backfills any pre-existing blank ones.
- games: home_team/away_team change from plain strings to {id, name,
  logo_url} objects -- home links to the actual Team (logo from the
  club's own logo, since teams have none of their own), away links
  to the actual Opponent (which already had a logo field). Breaking
  change for any existing consumer of the old string shape.
- sponsors: SponsorOut gains logo_width/logo_height, computed in
  Sponsor.save() -- Pillow for raster, a bounded regex read of the
  SVG root tag for vector logos (not a full XML parse, since that's
  exposed to entity-expansion attacks on untrusted uploads). A data
  migration backfills dimensions for existing sponsor logos.
- teams: PlayerOut gains `license`, sourced from ClubMembership (not
  Member -- it's per-club, per-season), batched in one query.

news/migrations/0002_backfill_slugs.py

@@ -0,0 +1,23 @@
+from django.db import migrations
+
+from rosterchief.base import unique_slugify
+
+
+def backfill_slugs(apps, schema_editor):
+    """News.save() already auto-populates a blank slug (rosterchief.base.
+    ClubScopedModel.save), but that only fires on save() -- rows created
+    before the slug field existed, or via bulk_create, never got one."""
+    News = apps.get_model("news", "News")
+    for item in News.objects.filter(slug=""):
+        item.slug = unique_slugify(item, item.title, scope={"club_id": item.club_id})
+        item.save(update_fields=["slug"])
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("news", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.RunPython(backfill_slugs, migrations.RunPython.noop),
+    ]