feat: add new subscription tier and update credit configurations

- Introduced a new "max" subscription tier with associated monthly credits and top-up limits.
- Updated existing subscription tiers' monthly credits and top-up limits for "starter", "pro", and "business".
- Enhanced credit display and overview components to reflect the new tier and its attributes.
- Integrated Polar authentication features for improved subscription management and credit handling.

lib/ai-models.ts

@@ -9,7 +9,7 @@ export interface AiModel {
   estimatedCost: string; // human-readable, e.g. "~€0.04"
   /** Credits pro Generierung — gleiche Einheit wie Convex reserve/commit (Euro-Cent). */
   creditCost: number;
-  minTier: "free" | "starter" | "pro" | "business"; // minimum subscription tier
+  minTier: "free" | "starter" | "pro" | "max" | "business"; // minimum subscription tier
 }
 
 export const IMAGE_MODELS: AiModel[] = [