N6/C3 W1: world boss — model, scaling, spawn + lifecycle

The monthly communal "Siege": a named boss camps outside town for 72h with
a single shared HP pool. This lands the model and the automatic lifecycle;
the player-facing bout command is W2.

- New tables world_boss + world_boss_contrib (own tables, outside the
  saveAdvCharacter fan-out, so a char save can't clobber the shared pool —
  the isolation adventure_shadow earns). Absent active row == no event; no
  bootstrap.
- Boss sized to the town it will fight: tier from the MEDIAN combined level
  of any-chat-active players (feedback_presence_is_any_chat, off
  daily_activity), pool HP = arena per-bout HP × ~2 bouts/active player,
  clamped [4,60] bouts. Floored at T3.
- Lifecycle rides the 1-min eventTicker (no net-new goroutine): auto-spawn
  on the 1st of each UTC month (JobCompleted dedup) + resolve a lapsed
  window as a survival. Operator override + the daily bout are W2.
- Resolution: defeat mints a bounty scaled by fights fought (not damage —
  accessibility) + a consumable cache + one low-rate treasure roll each;
  survival debits 20% of the community pot as a tribute (a pot sink). Both
  close-outs are guarded on status='active' so they fire once.
- Announcements post to the games room (no-op when GAMES_ROOM unset).

Pure logic (median, tier bucket, HP scaling, pool subtract/clamp, payout
split) is unit-tested; euro/DM emission left thin per the repo's no-client
-stub convention. Combat golden byte-identical (never touches SimulateCombat);
full plugin suite green.

Claude-Session: https://claude.ai/code/session_017mEwUmmS7aQTP2NQXj6rUa

internal/plugin/adventure_events.go

@@ -120,6 +120,10 @@ func (p *AdventurePlugin) eventTicker() {
 			// Reclaim invites nobody answered (N3/P6b). Every read already
 			// filters on the TTL; this just stops the rows accumulating.
 			purgeExpiredInvites()
+
+			// World boss (N6/C3): auto-spawn the monthly Siege on the 1st and
+			// resolve one whose 72h window has lapsed. Own dedup inside.
+			p.worldBossTick()
 		}
 	}
 }