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.sum

@@ -21,6 +21,8 @@ github.com/felixge/httpsnoop v1.0.4 h1:NFTV2Zj1bL4mc9sqWACXbQFVBBg2W3GPvqp8/ESS2
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@@ -29,6 +31,8 @@ github.com/gogo/protobuf v1.3.2 h1:Ov1cvc58UF3b5XjBnZv7+opcTcQFZebYjWzi34vdm4Q=
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@@ -64,13 +70,15 @@ github.com/pkg/errors v0.9.1 h1:FEBLx1zS214owpjy7qsBeixbURkuhQAwrK5UwLGTwt4=
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@@ -81,6 +89,8 @@ go.opentelemetry.io/otel/trace v1.29.0 h1:J/8ZNK4XgR7a21DZUAsbF8pZ5Jcw1VhACmnYt3
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