feat(playevents): add Writer with start/end/skipped/synthetic paths

Owns the auto-close-prior step (caps each user at one open row),
spec §6 skip classification rule on play_ended (AND of completion <
0.5 and duration_played < 30s), and skip_events row writes. Synthetic
completed play wires the Subsonic /rest/scrobble?submission=true path
into the same store as native events.

internal/db/dbq/users.sql.go

@@ -1,6 +1,6 @@
 // Code generated by sqlc. DO NOT EDIT.
 // versions:
-//   sqlc v1.27.0
+//   sqlc v1.31.1
 // source: users.sql
 
 package dbq