triage: clarify sentiment-overview 503 description (review feedback)

The 503 is reachable and genuinely PostgREST-related (authorizeOrg returns it when
the PostgREST client is absent, during brand->workspace resolution, before any
Semrush call), but "not configured for this deployment" read as misleading on a
Semrush-backed endpoint. Reword to state the actual condition and why.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: calvarezg

docs/index.html

@@ -1246,7 +1246,7 @@ v2-serenity-brand-presence-sentiment-overview:
           application/json:
             schema: { $ref: './schemas.yaml#/SerenityErrorResponse' }
       '503':
-        description: PostgREST client not configured for this deployment.
+        description: PostgREST client not available (required to resolve the brand's Semrush workspace before the element call).
         content:
           application/json:
             schema: { $ref: './schemas.yaml#/SerenityErrorResponse' }