fix(seo): add endDate, performer, offers to Event structured data

Clears the 3 non-critical Events issues in Search Console
("Missing field endDate/performer/offers") across all /projects/
recap pages. These are free, single-day events, so endDate mirrors
startDate, performer is MSOS (host org), and offers is a zero-price
Offer. Generated centrally in tools/seo_inject.py and re-injected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/seo_inject.py

@@ -431,12 +431,15 @@ def event_schema(lang, path, content, canon, desc, ogimg):
         "@context": "https://schema.org", "@type": "Event",
         "name": headline,
         "startDate": start,
+        "endDate": start,
         "eventStatus": "https://schema.org/EventScheduled",
         "eventAttendanceMode": "https://schema.org/OfflineEventAttendanceMode",
         "location": place,
         "image": ogimg,
         "inLanguage": HREF[lang],
         "organizer": {"@type": "Organization", "name": SITE_NAME[lang], "url": f"{DOMAIN}/{lang}/"},
+        "performer": {"@type": "Organization", "name": SITE_NAME[lang], "url": f"{DOMAIN}/{lang}/"},
+        "offers": {"@type": "Offer", "price": "0", "priceCurrency": "EUR", "availability": "https://schema.org/InStock", "url": canon, "validFrom": start},
         "url": canon,
     }
     if desc: