feat(db): migration 0003 - users.subsonic_password opt-in column

Subsonic token auth (t=md5(password+salt)) needs a reversibly stored
credential; bcrypt password_hash can't serve. NULL means the user has
not opted in, forcing apiKey (OpenSubsonic) instead.

internal/db/migrations/0003_subsonic_password.up.sql

@@ -0,0 +1,7 @@
+-- Subsonic's token auth (t = md5(password + salt)) requires the server to
+-- compute md5 over the user's plaintext password. bcrypt (password_hash) is
+-- not reversible, so we store a *separate*, opt-in, plaintext credential used
+-- exclusively for the /rest/* legacy auth path. NULL means the user has not
+-- opted into Subsonic t/s auth and must use apiKey (OpenSubsonic) instead.
+
+ALTER TABLE users ADD COLUMN subsonic_password text;