Add forex plugin, stability fixes, and async HTTP dispatch

- Add forex plugin (Frankfurter v2 API) with rate lookups, analysis,
  DM-based alerts, and daily cron poll. Backfills 1 year of history
  on startup for moving averages and buy signal scoring.

- Fix bot hang caused by SQLite lock contention in reminder polling:
  rows cursor was held open while writing to the same DB. Collect
  results first, close cursor, then process. Same fix in milkcarton.

- Add sync retry loop so the bot reconnects after network drops
  instead of silently exiting. StopSync() for clean Ctrl+C shutdown.

- Add panic recovery to all dispatch, syncer, and cron paths.

- Make all HTTP-calling plugin commands async (goroutines) so a slow
  or dead external API cannot block the message dispatch pipeline.
  Affects: lookup, stocks, forex, anime, movies, concerts, gaming,
  retro, wotd, urls, howami.

- Extract DisplayName to Base, add db.Exec helper, convert silent
  error discards across the codebase.

- Fix UNO mercy-kill bug (eliminated bot continues playing), adventure
  DM nag spam, stats column mismatch, per-call regex/replacer allocs.

- Update README: forex commands, Finance section, 47 plugins.

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

internal/plugin/adventure_treasure.go

@@ -281,6 +281,7 @@ func advSaveTreasure(userID id.UserID, def *AdvTreasureDef) error {
 	if err != nil {
 		return err
 	}
+	defer tx.Rollback()
 
 	for _, bonus := range def.Bonuses {
 		_, err := tx.Exec(`
@@ -288,7 +289,6 @@ func advSaveTreasure(userID id.UserID, def *AdvTreasureDef) error {
 			VALUES (?, ?, ?, ?, ?, ?)`,
 			string(userID), def.Key, def.Name, def.Tier, bonus.Type, bonus.Value)
 		if err != nil {
-			tx.Rollback()
 			return err
 		}
 	}