feat: migrate file storage to postgres with sqlx

backend/migrations/20260430221144_create_file_records.sql

@@ -0,0 +1,7 @@
+CREATE TABLE file_records (
+    id          BIGSERIAL    PRIMARY KEY,
+    name        TEXT         NOT NULL,
+    file_type   TEXT         NOT NULL,
+    size        BIGINT       NOT NULL,
+    uploaded_at TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+);