Add DATABASE_URL env var support for EmDash

- Add .env.example documenting SQLite/PostgreSQL/MySQL config
- Add src/lib/db.ts with getDatabaseConfig() helper that parses
  DATABASE_URL and returns the appropriate db config
- Update astro.config.mjs to use getDatabaseConfig() instead of
  hardcoded sqlite path

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

astro.config.mjs

@@ -3,7 +3,7 @@ import react from "@astrojs/react";
 import { auditLogPlugin } from "@emdash-cms/plugin-audit-log";
 import { defineConfig, fontProviders } from "astro/config";
 import emdash, { local } from "emdash/astro";
-import { sqlite } from "emdash/db";
+import { getDatabaseConfig } from "./src/lib/db.ts";
 
 export default defineConfig({
 	output: "server",
@@ -17,7 +17,7 @@ export default defineConfig({
 	integrations: [
 		react(),
 		emdash({
-			database: sqlite({ url: "file:./storage/data.db" }),
+			database: getDatabaseConfig(),
 			storage: local({
 				directory: "./storage/uploads",
 				baseUrl: "/_emdash/api/media/file",