feat: implement Files page with persistent storage and version tracking

Three-panel file manager (nav, list, details) modeled after Stirling-PDF.
- Backend: user_files table, /api/v1/files/* CRUD routes, file storage
  helpers, thumbnail generation via Sharp, recursive CTE version chains
- Frontend: FilesNav, FileList, FileDetails, FileUploadArea components,
  Zustand store, mobile layout with bottom sheet
- Integration: tool-factory auto-saves results as new versions when
  fileId is provided, "Open File" loads file into tool processing flow
- Search, bulk select/delete/download, version badges, tool chain tags

apps/api/src/db/index.ts

@@ -1,9 +1,9 @@
+import { mkdirSync } from "node:fs";
+import { dirname } from "node:path";
 import Database from "better-sqlite3";
 import { drizzle } from "drizzle-orm/better-sqlite3";
 import { env } from "../config.js";
 import * as schema from "./schema.js";
-import { mkdirSync } from "node:fs";
-import { dirname } from "node:path";
 
 // Ensure data directory exists
 mkdirSync(dirname(env.DB_PATH), { recursive: true });
@@ -17,4 +17,4 @@ sqlite.pragma("synchronous = NORMAL");
 sqlite.pragma("foreign_keys = ON");
 
 export const db = drizzle(sqlite, { schema });
-export { schema };
+export { schema, sqlite };