add migration system

backend/internal/db/migrations/001_init.down.sql

@@ -0,0 +1 @@
+-- Rollback 001_init

backend/internal/db/migrations/001_init.up.sql

@@ -0,0 +1,3 @@
+-- VaultDrop 001: Initial schema
+-- This is a placeholder. Add your first CREATE TABLE here.
+SELECT 1;

backend/internal/db/migrations/schema.sql

@@ -1,20 +0,0 @@
-CREATE TABLE files (
-  id TEXT PRIMARY KEY,
-  name TEXT NOT NULL,
-  storage_key TEXT NOT NULL,
-  checksum TEXT NOT NULL,
-  created_at INTEGER NOT NULL,
-  updated_at INTEGER NOT NULL,
-  deleted_at INTEGER
-);
-
-CREATE TABLE tags (
-  id TEXT PRIMARY KEY,
-  name TEXT NOT NULL
-);
-
-CREATE TABLE file_tags (
-  file_id TEXT NOT NULL,
-  tag_id TEXT NOT NULL,
-  PRIMARY KEY (file_id, tag_id)
-);