feat: add stickers

migrations/0054_stickers.up.sql

@@ -0,0 +1,43 @@
+-- Stickers in chat.
+--
+-- A sticker IS a sādhana reward — same catalog, same artwork — so there is no
+-- separate sticker table. What's new is a second way to come by one: someone
+-- sends you theirs and you keep it.
+--
+-- saved_stickers records only that second route. A reward you earned is
+-- already yours through sadhna_reward_grants, and duplicating it here would
+-- give two answers to "do I own this?".
+CREATE TABLE IF NOT EXISTS saved_stickers (
+    user_id    UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    reward_id  UUID        NOT NULL REFERENCES sadhna_rewards(id) ON DELETE CASCADE,
+    -- Who sent it. Kept for provenance, and SET NULL rather than CASCADE so
+    -- deleting an account doesn't take away stickers other people saved.
+    saved_from UUID        REFERENCES users(id) ON DELETE SET NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, reward_id)
+);
+
+-- "What's in my sticker drawer?" — the whole reason this table exists.
+CREATE INDEX IF NOT EXISTS saved_stickers_user_idx
+    ON saved_stickers (user_id, created_at DESC);
+
+-- A sticker message: kind 'sticker', with the artwork denormalized onto the
+-- attachment's existing image_url/title columns.
+--
+-- Denormalized rather than resolved per read, unlike a shared post: the
+-- catalog is static art with no viewer-dependent visibility, so there is
+-- nothing to re-check and a join on every message would buy nothing.
+ALTER TABLE message_attachments DROP CONSTRAINT IF EXISTS message_attachments_kind_check;
+
+ALTER TABLE message_attachments
+    ADD CONSTRAINT message_attachments_kind_check
+    CHECK (kind IN ('image', 'video', 'audio', 'file', 'link', 'entity', 'quote', 'sticker'));
+
+-- entity_id carries the reward key on a sticker attachment, so a recipient can
+-- save it without the client having to trust a URL it was handed.
+ALTER TABLE message_attachments DROP CONSTRAINT IF EXISTS message_attachments_entity_type_check;
+
+ALTER TABLE message_attachments
+    ADD CONSTRAINT message_attachments_entity_type_check
+    CHECK (entity_type IS NULL OR
+           entity_type IN ('post', 'comment', 'profile', 'collection', 'sticker'));