chore: ZXDB env validation, MySQL setup, API & UI

This sanity commit wires up the initial ZXDB integration and a minimal UI to explore it.

Key changes:
- Add Zod-based env parsing (`src/env.ts`) validating `ZXDB_URL` as a mysql:// URL (t3.gg style).
- Configure Drizzle ORM with mysql2 connection pool (`src/server/db.ts`) driven by `ZXDB_URL`.
- Define minimal ZXDB schema models (`src/server/schema/zxdb.ts`): `entries` and helper `search_by_titles`.
- Implement repository search with pagination using helper table (`src/server/repo/zxdb.ts`).
- Expose Next.js API route `GET /api/zxdb/search` with Zod query validation and Node runtime (`src/app/api/zxdb/search/route.ts`).
- Create new app section “ZXDB Explorer” at `/zxdb` with search UI, results table, and pagination (`src/app/zxdb/*`).
- Add navbar link to ZXDB (`src/components/Navbar.tsx`).
- Update example.env with readonly-role notes and example `ZXDB_URL`.
- Add drizzle-kit config scaffold (`drizzle.config.ts`).
- Update package.json deps: drizzle-orm, mysql2, zod; devDeps: drizzle-kit. Lockfile updated.
- Extend .gitignore to exclude large ZXDB structure dump.

Notes:
- Ensure ZXDB data and helper tables are loaded (see `ZXDB/scripts/ZXDB_help_search.sql`).
- This commit provides structure-only browsing; future work can enrich schema (authors, labels, publishers) and UI filters.

Signed-off-by: Junie@lucy.xalior.com

src/app/zxdb/page.tsx

@@ -0,0 +1,9 @@
+import ZxdbExplorer from "./ZxdbExplorer";
+
+export const metadata = {
+  title: "ZXDB Explorer",
+};
+
+export default function Page() {
+  return <ZxdbExplorer />;
+}