feat: `tuitube sync --watch` background daemon for auto-syncing subscribed stations

[gitcoder89431]

Feature

A --watch flag for tuitube sync that runs as a background daemon, polling subscribed stations on a configurable interval and syncing new tracks automatically — no manual tuitube sync required.

Why

Right now, tuitube is a snapshot: you sync once and your library ages. For daily-driver use, users want new uploads from their lofi/jazz/ambient channels to appear automatically — like a podcast client or RSS reader. This is the feature that turns tuitube from a "tool I have to maintain" into a "service that just works."

UX

# Run in background, syncs every 6 hours
tuitube sync --watch --interval 6h

# Or managed by systemd/launchd
# tuitube provides a `tuitube install-service` command that writes the unit file

When new tracks are found:

• Write to DB as normal
• Optionally send a desktop notification via notify-send (Linux) or osascript (macOS) — gated by a config flag

Implementation sketch

1. Parse --interval as time.Duration (default 6h)
2. Run SyncStations() immediately on start, then time.Sleep(interval) in a loop
3. PID file at ~/.local/share/tuitube/sync.pid to prevent double-start
4. Graceful shutdown on SIGTERM/SIGINT
5. tuitube sync --stop to kill the daemon
6. tuitube install-service subcommand writes:
   • ~/.config/systemd/user/tuitube-sync.service on Linux
   • ~/Library/LaunchAgents/com.tuitube.sync.plist on macOS

Config integration (see #38)

[sync]
poll_interval = "6h"
notify_on_new_tracks = true

Files likely affected

• cmd/sync.go — add --watch, --interval, --stop flags
• New internal/daemon/daemon.go for PID management + signal handling
• New cmd/installservice.go for the service file writer