feat: contract types for Post reactions (ApiPostReaction*)

[arturasmckwcz]

Context

need4deed-org/be#928 needs reaction support on Posts and replies for the Posts page rebuild (need4deed-org/fe#969 epic). Per shared-rules.md, the API contract must be defined here first.

Scope

• ApiPostReactionCreate — target reference (targetType: 'post' | 'reply', targetId) + emoji
• ApiPostReactionGet — aggregated per-emoji counts on a post/reply, plus whether the requesting user reacted
• Open question to resolve before finalizing the shape: one reaction per person per item, or multiple simultaneous reactions allowed (affects whether emoji is singular or a list per person)

Acceptance criteria

• Reaction cardinality question resolved with product
• Types added and exported from the SDK
• Version bumped and published to npmjs.com

Blocks: need4deed-org/be#928
Depends on reply id shape from: #213 — reactions can target a reply
Referenced from need4deed-org/fe#969