feat: chat reactions

migrations/0053_message_reactions.up.sql

@@ -0,0 +1,27 @@
+-- Emoji reactions on chat messages.
+--
+-- One reaction per person per message: reacting again REPLACES rather than
+-- accumulates, which is what the primary key enforces. That matches how the
+-- UI reads — a row of distinct emoji with counts — and stops someone stacking
+-- twenty reactions on one bubble.
+--
+-- The emoji is stored as text rather than a code point or shortname: what the
+-- sender picked is what everyone should see, and a mapping table would need
+-- updating every time the emoji set grows.
+CREATE TABLE IF NOT EXISTS message_reactions (
+    message_id      UUID        NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
+    user_id         UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    -- Denormalized so deleting a conversation takes its reactions with it
+    -- without relying on the messages cascade alone.
+    conversation_id UUID        NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
+    emoji           VARCHAR(32) NOT NULL,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (message_id, user_id),
+    -- A stray empty string would render as an invisible reaction chip.
+    CONSTRAINT message_reactions_emoji_not_blank CHECK (btrim(emoji) <> '')
+);
+
+-- Hydrating a page of messages: one index range scan per batch.
+CREATE INDEX IF NOT EXISTS message_reactions_message_idx
+    ON message_reactions (message_id);