fix(auth): the Subsonic password is generated, never the login password (M462 #5026)

`minstrel admin reset-password` copied the new login password into
subsonic_password, which is stored in plain text because Subsonic t/s
sign-in needs it. Every account recovered through the CLI had its login
password readable in the database, and changing the password later left
the copy behind.

- reset-password now changes only password_hash.
- Migration 0064 clears every subsonic_password, removing the copies.
- Settings gets a Subsonic password card: the server generates a random
  password, shows it once, and it can be regenerated or turned off
  (GET/POST/DELETE /api/me/subsonic-password, audited). Generated rather
  than user-chosen so it can never be a reused password.
- docs/security.md describes the separate password instead of the known
  issue.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/audit/audit.go

@@ -60,6 +60,10 @@ const (
 	// and its history moved onto the copy kept. The metadata names both, so the
 	// log can answer "where did that file go" long after the report is gone.
 	ActionDuplicateMerge Action = "duplicate_merge"
+
+	// Subsonic password (#5026): generated in Settings for t/s-only clients.
+	ActionSubsonicPasswordSet   Action = "subsonic_password_set"
+	ActionSubsonicPasswordClear Action = "subsonic_password_clear"
 )
 
 // Write inserts one audit_log row. metadata is marshaled as JSON;