Phase 2: Citation dashboard endpoint

[neuromechanist]

Part of epic #321

Scope

Expose canonical-paper citation tracking as a public, read-only JSON feed with a per-year and stacked-by-canonical-paper breakdown, gated per-community.

Tasks

• Add cites_doi column to the papers table schema (src/knowledge/db.py)
• Persist cites_doi in sync_citing_papers (DOI is known per-citation at fetch time, currently discarded)
• Full citation re-sync to backfill the linkage (cheap)
• Gate exposure on public_feeds.citations
• GET /{community_id}/citations returning per_year counts plus by_paper stacked breakdown keyed by canonical DOI
• Tests against real dev data

Depends on the public_feeds config block added in Phase 1.