feat: compute asset sha256 for dedupe

packages/db/migrations/0007_asset_hashes.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS asset_hashes (
+  id uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  asset_id uuid NOT NULL REFERENCES assets(id) ON DELETE CASCADE,
+  bucket text NOT NULL,
+  sha256 text NOT NULL,
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS asset_hashes_asset_id_idx ON asset_hashes(asset_id);
+
+CREATE UNIQUE INDEX IF NOT EXISTS asset_hashes_bucket_sha256_idx
+ON asset_hashes(bucket, sha256) WHERE sha256 IS NOT NULL;