feat(auth): store Subsonic API keys hashed; a new key is shown once (M462 #4983)

users.api_token held each user's apiKey in plaintext and was looked up by
equality, so a leaked row or backup handed out working keys. Migration
0063 replaces it with api_token_hash (sha256, hex), computed in place
from the existing keys so every Subsonic client keeps working.

The key can no longer be read back: GET /api/me/api-token is gone, and
POST returns the new key once. Settings shows it right after Regenerate
with a copy button and a "won't be shown again" note.

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

internal/subsonic/auth.go

@@ -36,7 +36,7 @@ func UserFromContext(ctx context.Context) (dbq.User, bool) {
 	return u, ok
 }
 
-// Middleware authenticates the request against users.api_token (apiKey) or
+// Middleware authenticates the request against users.api_token_hash (apiKey) or
 // users.subsonic_password (t/s or p). On failure it writes a Subsonic failed
 // envelope using the request's f= format; downstream handlers never see an
 // unauthenticated request.
@@ -79,7 +79,7 @@ func authenticate(r *http.Request, pool *pgxpool.Pool, cfg Config) (dbq.User, in
 	params := r.URL.Query()
 
 	if apiKey := params.Get("apiKey"); apiKey != "" {
-		user, err := q.GetUserByAPIToken(r.Context(), apiKey)
+		user, err := q.GetUserByAPITokenHash(r.Context(), auth.HashAPIToken(apiKey))
 		if err != nil {
 			if errors.Is(err, pgx.ErrNoRows) {
 				return dbq.User{}, ErrWrongCredentials, "Invalid apiKey"