feat: add asset variants table and URL selection

packages/db/migrations/0003_asset_variants.sql

@@ -0,0 +1,20 @@
+CREATE TYPE IF NOT EXISTS asset_variant_kind AS ENUM (
+  'thumb',
+  'poster',
+  'video_mp4'
+);
+
+CREATE TABLE IF NOT EXISTS asset_variants (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  asset_id uuid NOT NULL REFERENCES assets(id) ON DELETE CASCADE,
+  kind asset_variant_kind NOT NULL,
+  size int NOT NULL,
+  key text NOT NULL,
+  mime_type text NOT NULL,
+  width int,
+  height int,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  UNIQUE(asset_id, kind, size)
+);
+
+CREATE INDEX IF NOT EXISTS asset_variants_asset_id_idx ON asset_variants(asset_id);