chore(membership): lower image storage quotas for free and pro tiers

Reduce image_storage_bytes from 500MB to 25MB on free and from 1GB to
500MB on pro to align quotas with current storage cost expectations.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/configs/config.yaml

@@ -104,7 +104,7 @@ membership:
       article_quota_cycle: monthly
       ai_points_monthly: 1000
       ai_point_base_chars: 1000
-      image_storage_bytes: 524288000
+      image_storage_bytes: 26214000
       company_limit: 1
       subscription_duration: 720h
     - code: pro
@@ -113,7 +113,7 @@ membership:
       article_quota_cycle: monthly
       ai_points_monthly: 1500
       ai_point_base_chars: 1000
-      image_storage_bytes: 1073741824
+      image_storage_bytes: 524288000
       company_limit: 2
       subscription_duration: 720h