feat(playlists): wire Refresh into PUT /api/me/timezone + registration

Completes the server side of #392 Half B.

PUT /api/me/timezone now calls scheduler.Refresh(ctx, userID) after
the DB write so the rescheduled daily-at-03:00-local job takes
effect synchronously. Failure to refresh is logged but doesn't
undo the DB write — the hourly reconciliation pass would pick it
up within an hour regardless.

POST /api/auth/register calls Refresh after successful user
insert so brand-new users get scheduled immediately rather than
waiting for the hourly pass to discover them.

system_cron.go deleted: the new scheduler subsumes its
responsibilities. The StartSystemPlaylistCron call in main.go is
also removed. Server restart now runs the new scheduler's startup
recovery + catch-up instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/playlists/system_cron.go

@@ -1,61 +0,0 @@
-package playlists
-
-import (
-	"context"
-	"log/slog"
-	"time"
-
-	"github.com/jackc/pgx/v5/pgxpool"
-
-	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
-)
-
-// StartSystemPlaylistCron launches the in-process daily build loop in a
-// goroutine. Caller passes a context that's cancelled on graceful shutdown.
-//
-// Lifecycle:
-//  1. Clear any stale in_flight=true rows from a previously crashed process
-//     (safe at startup: by definition, nothing is running yet).
-//  2. Run once at startup so users catch up after a server restart without
-//     waiting up to 24h.
-//  3. Tick every 24h; runOnce on each tick.
-//  4. Exit on context cancellation.
-//
-// Per-user errors are logged but don't halt the loop. Per-cycle errors (e.g.,
-// "list active users" failed) are logged but the next tick still fires.
-func StartSystemPlaylistCron(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string) {
-	go func() {
-		q := dbq.New(pool)
-		if err := q.ClearStaleSystemPlaylistInFlight(ctx); err != nil {
-			logger.Warn("system playlist cron: startup recovery failed", "err", err)
-		}
-
-		runOnce(ctx, pool, logger, dataDir)
-
-		ticker := time.NewTicker(24 * time.Hour)
-		defer ticker.Stop()
-		for {
-			select {
-			case <-ctx.Done():
-				return
-			case <-ticker.C:
-				runOnce(ctx, pool, logger, dataDir)
-			}
-		}
-	}()
-}
-
-func runOnce(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string) {
-	q := dbq.New(pool)
-	users, err := q.ListActiveUsersForSystemPlaylists(ctx)
-	if err != nil {
-		logger.Error("system playlist cron: list active users failed", "err", err)
-		return
-	}
-	for _, userID := range users {
-		if err := BuildSystemPlaylists(ctx, pool, logger, userID, time.Now(), dataDir); err != nil {
-			logger.Warn("system playlist build failed in cron",
-				"user_id", uuidStringPL(userID), "err", err)
-		}
-	}
-}