feat(playlists): gocron-based per-user scheduler

Builds the per-user daily-at-03:00-local scheduler for #392 Half B.
Uses github.com/go-co-op/gocron/v2 with WithLocation(userTZ) for
each user's job. Hourly reconciliation pass keeps the in-memory
job set in sync with the active-users query.

Start sequence: clear stale in_flight rows; register a daily job
for every active user at their stored timezone; fire a one-shot
runOnce to catch up missed schedules during downtime; start gocron.

Stop drains and shuts down the gocron loop.

Refresh(ctx, userID) removes the user's existing job (if any) and
registers a fresh one at their current timezone — wired into the
PUT /api/me/timezone handler and new-user registration in the next
commit.

Server struct gains PlaylistScheduler; main.go constructs it after
the eventbus and threads it through to api.Mount. The existing
StartSystemPlaylistCron call stays for one more commit so we don't
have a window where no scheduler is running; Task 3 deletes it.

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

go.mod

@@ -5,10 +5,12 @@ go 1.23.0
 require (
 	github.com/dhowden/tag v0.0.0-20240417053706-3d75831295e8
 	github.com/go-chi/chi/v5 v5.2.5
+	github.com/go-co-op/gocron/v2 v2.21.2
 	github.com/golang-migrate/migrate/v4 v4.18.2
+	github.com/google/uuid v1.6.0
 	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa
 	github.com/jackc/pgx/v5 v5.7.4
-	github.com/stretchr/testify v1.9.0
+	github.com/stretchr/testify v1.11.1
 	golang.org/x/crypto v0.35.0
 	gopkg.in/yaml.v3 v3.0.1
 )
@@ -20,7 +22,9 @@ require (
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/jonboulle/clockwork v0.5.0 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/robfig/cron/v3 v3.0.1 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	go.uber.org/atomic v1.7.0 // indirect
 	golang.org/x/sync v0.11.0 // indirect