feat: emoji reactions on Posts and replies

[arturasmckwcz]

Context

Prototype feedback for fe#892 added emoji reactions (a quick-option bar plus a full searchable picker) on posts and replies. No reaction concept exists in the backend today.

Blocked on need4deed-org/sdk#214 (contract-first per shared-rules.md) and #927 (reactions can target a reply, so the Reply entity must exist first).

Scope

• New PostReaction entity: target reference (post or reply), person, emoji (short unicode string)
• Open question to confirm with product before migrating: one reaction per person per item, or multiple simultaneous reactions per person allowed? Affects the unique constraint (tracked in sdk#214)
• Endpoints: POST/DELETE /post/:id/reaction and the reply equivalent; GET responses include aggregated per-emoji counts plus whether the requesting user reacted
• Implement against the types published from feat: contract types for Post reactions (ApiPostReaction*) sdk#214

Acceptance criteria

• feat: contract types for Post reactions (ApiPostReaction*) sdk#214 published and this depends on the published version (not a local/linked SDK)
• Reaction cardinality (single vs multiple per person) confirmed with product before writing the migration
• Schema change confirmed necessary/semantically correct (per shared-rules.md)
• Reaction endpoints exist and documented in swagger
• Migration replays cleanly on a fresh DB

Blocked on: need4deed-org/sdk#214, #927
Referenced from need4deed-org/fe#969