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/db/queries/users.sql

@@ -36,7 +36,8 @@ SELECT count(*) FROM users;
 
 -- name: SetSubsonicPassword :exec
 -- Stores (or clears with NULL) the per-user Subsonic legacy credential used
--- for t/s and p auth on /rest/*. Must be plaintext; see migration 0003.
+-- for t/s and p auth on /rest/*. Must be plaintext; see migration 0003. Only
+-- ever a server-generated value, never the login password (#5026).
 UPDATE users SET subsonic_password = $2 WHERE id = $1;
 
 -- name: GetUserByID :one