fix: add credibility_tier to per-source entries in person detail enrichment

[krisoye]

Context

GET /persons/{person_id} enriches each source with {source_id, role, title} (kb_server.py lines 2088-2105). The per-source credibility_tier is collected to compute the person-level tier but is NOT added back to individual source entries.

Problem

The admin-dashboard PersonDetailPage.tsx reads src.credibility_tier on each source row — always undefined, always falls back to "unverified".

Fix

In kb_server.py enrichment loop (~line 2098), after fetching source metadata, add credibility_tier to the enriched source dict:

enriched["credibility_tier"] = tier  # from metadata.get("credibility_tier")

Acceptance Criteria

• GET /persons/{person_id} sources array includes credibility_tier per source
• Sources without a tier return null or omit the field

Part of krisoye/admin-dashboard#72