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/countdown.go

@@ -260,13 +260,9 @@ func (p *CountdownPlugin) handleShow(ctx MessageContext, idStr string) error {
 }
 
 func (p *CountdownPlugin) completeOldCountdowns() {
-	d := db.Get()
 	cutoff := time.Now().UTC().AddDate(0, 0, -7).Format("2006-01-02")
-	_, err := d.Exec(
+	db.Exec("countdown: auto-complete",
 		`UPDATE countdowns SET completed = 1 WHERE target_date < ? AND completed = 0`,
 		cutoff,
 	)
-	if err != nil {
-		slog.Error("countdown: auto-complete", "err", err)
-	}
 }