feat: add location and categories fields, simplify event creation

- Add location (text) and categories (JSON list) to Event model
- Make most CreateEvent fields optional: only title + start date required
- Default end_date to start_date, closing_date to end_date
- Categories stored as JSON text, parsed via validator
- NIP-52 publisher includes location tag and t tags for categories
- Migration m011 adds location and categories columns

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

nostr_publisher.py

@@ -40,13 +40,17 @@ def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
         tags.append(["end", event.event_end_date])
     if event.banner:
         tags.append(["image", event.banner])
+    if event.location:
+        tags.append(["location", event.location])
+    for cat in (event.categories or []):
+        tags.append(["t", cat])
 
     nostr_event = NostrEvent(
         pubkey=pubkey,
         created_at=int(time.time()),
         kind=31922,
         tags=tags,
-        content=event.info,
+        content=event.info or "",
     )
     nostr_event.id = nostr_event.event_id
     return nostr_event