Implement FMSG-005 reactions and the terminal flag

[markmnl]

Summary

Implements FMSG-005 Reactions and exposes the fmsg Specification v0.6.0 terminal flag. Requires fmsgd's msg.is_terminal column from markmnl/fmsgd#43.

Message objects

• terminal: the fmsg terminal flag, settable on POST /fmsg and PUT /fmsg/:id.
• reaction: the emoji a message carries when it is a reaction ("" for a clearing reaction), null otherwise, so clients can hide reaction messages from lists.
• reactions: effective reactions on the message. Each participant's latest reaction by time, ties broken by message hash, grouped by emoji in order of first reaction.

Recognition is by FMSG-005 shape: a reply with no_reply and terminal set, not important, no add-to, type text/plain;charset=UTF-8, no attachments, body a single emoji or empty. No dedicated media type.

POST /fmsg/:id/react sets or clears the caller's reaction by creating and sending a reaction message to every other participant in one transaction. Idempotent when the reaction is unchanged. Applies the same reply-domain check as send. Local recipients get their delivery resolved as send does.

Terminal enforcement at the API: a reply to a terminal message (create/update) and add-to on one return 409 with a clear message rather than surfacing the database trigger error.

WebSocket and push: an arriving reaction is routed as a reaction event carrying the refreshed subject message, never as new_msg, and produces no Web Push.

internal/emoji: IsRGI (exact RGI_Emoji membership), IsPossible (UTS #51 possible_emoji grammar with emoji presentation required) and IsSingle. Tables generated from Unicode 17.0 by go run ./internal/emoji/gen, which downloads the latest data by default.

Thread routes are unaffected: they walk ancestors, and reactions are leaves. terminal is added to thread message objects.

Verification

• go build, go vet, go test ./... pass, with new tests for shape recognition, data reading, effective-reaction reduction (change, clear, case-insensitive reactor, hash tie-break), recipient derivation, and hub routing of reactions to the subject.
• The new SQL (reaction insert, loadReactionRows, reactionSubject, list selects) was executed against fmsgd's v0.6.0 dd.sql plus this repo's dd.sql in a scratch PostgreSQL 16, and the schema trigger refused a reply to the reaction.

Docs

• README: route table, POST /fmsg and PUT fields and errors, message object fields, new react section, add-to 409, WebSocket event table.
• FMSG-003 v0.2.0 in the spec PR documents the same contract.

Follow-ups

• fmsg-docker: integration test for react, and sync fmsgd's dd.sql copy once fmsgd#43 merges.
• fmsg-cli / hermes-fmsg: expose the react route.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MW4FXDGacoWXeKPU5cQTHS