fix: image generation 503 when Firebase Admin is not configured

[Copilot]

All four image routes unconditionally called authenticateFirebaseUser, which throws 503 when adminAuth is null (Firebase Admin SDK unconfigured). This made character/board/background-removal image generation entirely unavailable in any deployment without Firebase Admin credentials, even when FAL_KEY is set.

Changes

• server/routes/images.js — Accept adminAuth as a parameter; guard auth calls behind if (adminAuth). For /api/generate-board-image and /api/board-image-status (which use caller.uid for in-memory job tracking), fall back to { uid: 'anonymous' } when adminAuth is absent.
• server/index.js — Pass adminAuth into registerImageRoutes.

// Before — always throws 503 when adminAuth is null
await authenticateFirebaseUser(req);

// After — gracefully skips verification when Firebase Admin is unconfigured
if (adminAuth) await authenticateFirebaseUser(req);

Follows the same graceful-degradation pattern already used in the admin and battle route registrations (both guard their auth-dependent paths with if (!adminAuth)).