Reactions on Journal and Register entries

What ships in this project

• Postgres reactions table with entry_kind, entry_slug, kind enum (support / concern), user_id and client_token columns under XOR check, partial unique indexes, and RLS policies.
• Anonymous client token stored in an httpOnly cookie (oln_anon_id, 1-year max-age) so anonymous reactions remain attributable to a single browser without identifying the user.
• Server actions (reactAction, unreactAction) returning a discriminated ActionResult so the client widget can rollback on rejection and lock the UI for write-once anonymous reactions.
• Client widget (ReactionButtons) with optimistic apply/rollback, retract on second click for registered users, and an aria-pressed selected state.
• Mounted on /journal/[slug] and /register/[slug] for parity.

Why

Entry 015 commits to a reaction primitive in principle. This project implements the thin slice end-to-end so the priority signal exists before the roadmap surface needs it (Entry 016).

Open follow-ups

• G-036 — operational parameters (anonymous-vs-registered weighting, cooldowns, abuse heuristics).
• Aggregation views beyond per-entry counts (e.g., a "most reacted" surface).