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/auth/session.go

@@ -5,6 +5,7 @@ import (
 	"crypto/rand"
 	"crypto/sha256"
 	"encoding/base64"
+	"encoding/hex"
 	"errors"
 	"log/slog"
 	"net/http"
@@ -41,6 +42,15 @@ func HashSessionToken(token string) []byte {
 	return sum[:]
 }
 
+// HashAPIToken maps a raw API key (the OpenSubsonic apiKey) to the
+// `users.api_token_hash` column: sha256, hex. The key is minted with
+// MintSessionToken, so it carries the same 256 bits and the same reasoning
+// as HashSessionToken applies. Hex rather than bytes so the column stays
+// text and a migration can compute it in SQL from the old plaintext.
+func HashAPIToken(token string) string {
+	return hex.EncodeToString(HashSessionToken(token))
+}
+
 // VerifyPassword is the canonical bcrypt comparison. Returns false on a
 // malformed hash so callers don't need to distinguish "hash invalid" from
 // "password wrong" — both are auth failures from the client's perspective.