Add market plugin, arena, holidays, UNO stacking fix, and multiple UX improvements

- Market plugin (#49): daily index snapshots (Yahoo Finance + Finnhub fallback),
  Ollama-generated summaries, !howsthemarket, !marketstatus, !marketreport commands,
  exchange hours with DST support, 30-min room cooldown
- Adventure arena: 5-tier combat gauntlet with 20 monsters, risk-reward cashout,
  death lockout changed from 24h to midnight UTC across all code and flavor text
- Adventure holidays: double daily actions on holidays
- Adventure DM fix: 15-minute response window prevents bare numbers from triggering
  adventure during UNO games
- Adventure scheduler: jitter between morning DMs to avoid Matrix rate limits
- Adventure revive: wire up adv_revived achievement on admin revive
- Column migration system for existing databases (holiday_action_taken)
- Fix italic markdown rendering after newlines
- Fix holdem DMs broken by space groups including DM rooms
- UNO No Mercy: remove stacking escalation rule (any draw card stacks on any other)
- UNO multiplayer: add turn announcements after stack absorption and turn passes,
  jitter between rapid-fire messages with safe mutex handling
- Birthday: add €1,000 gift and 10 XP + €100 community celebration bonus
- Market: guard against division by zero, nil pointer, and timezone errors
- README updates: plugin count 48→49, all new commands, feature flags, architecture

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/plugin/uno.go

@@ -744,8 +744,8 @@ func (p *UnoPlugin) handlePlayerPlay(game *unoGame, idx int) error {
 	if game.noMercy && game.stackMinValue > 0 {
 		if !card.canPlayOnStacking(game.topColor, game.stackMinValue) {
 			return p.SendMessage(game.dmRoomID,
-				fmt.Sprintf("You can't stack %s — need a draw card worth +%d or more. Type **accept** to draw %d.",
-					card.Display(), game.stackMinValue, game.stackTotal))
+				fmt.Sprintf("You can't stack %s — need a draw card (matching color or wild). Type **accept** to draw %d.",
+					card.Display(), game.stackTotal))
 		}
 	} else if !card.canPlayOn(game.discardTop, game.topColor) {
 		return p.SendMessage(game.dmRoomID, fmt.Sprintf("You can't play %s on %s. Choose another card or **draw**.",