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_test.go

@@ -216,7 +216,7 @@ func TestCanPlayOnStacking(t *testing.T) {
 		{"draw two on draw two same color", unoCard{unoRed, unoDrawTwo}, unoRed, 2, true},
 		{"draw two wrong color", unoCard{unoBlue, unoDrawTwo}, unoRed, 2, false},
 		{"number cant stack", unoCard{unoRed, unoFive}, unoRed, 2, false},
-		{"draw two cant stack on 4", unoCard{unoRed, unoDrawTwo}, unoRed, 4, false},
+		{"draw two stacks on 4 same color", unoCard{unoRed, unoDrawTwo}, unoRed, 4, true},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {