fix(viewer): detect DB changes under WAL so new assets load - #3062

Problem

On a freshly-flashed x86 testbed running master, adding the first asset left the viewer stuck on the standby screen, logging Playlist is empty forever — even though the asset was enabled, active, and visible to both the server and the viewer via the ORM. More broadly, the viewer's polling-based playlist refresh stopped detecting asset edits.

Root cause

The viewer decides when to reload its playlist by polling the database file's mtime, stat'ing only the main anthias.db file:

def get_db_mtime(self) -> float:
    try:
        return path.getmtime(settings['database'])
    except (OSError, TypeError):
        return 0

Since #3015 the DB is opened with PRAGMA journal_mode=WAL. Under WAL, commits are written to the anthias.db-wal sidecar (and update anthias.db-shm); the main file's mtime stays frozen until a checkpoint, which during normal operation is infrequent. So get_db_mtime() never advanced and refresh_playlist() never reloaded.

The fallbacks don't save it: an empty starting playlist yields a None deadline (_compute_deadline returns None with no candidates), so the deadline path can't recover the first asset; and for non-empty playlists the deadline is the soonest end_date, typically far in the future.

This is a regression from #3015 — the viewer's change-detection wasn't updated for WAL's sidecar files.

Fix

Make get_db_mtime() WAL-aware: take the newest mtime across anthias.db, anthias.db-wal, and anthias.db-shm. WAL commits move the sidecars and a checkpoint moves the main file, so the value advances on every write regardless of journal mode. This keeps the existing cheap filesystem-stat design (no per-loop DB query); PRAGMA data_version was considered but rejected for that reason.

Testing

• New regression test test_get_db_mtime_tracks_wal_sidecar: a write touching only the -wal sidecar is now detected.
• Full non-integration suite green (1087 passed).
• Verified live on the x86 testbed: before the fix, touch ~/.anthias/anthias.db was required to make the first asset appear; with WAL-aware detection the viewer picks it up on its own.

Fixes #3061