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>

src/lib/db.ts

@@ -0,0 +1,37 @@
+import { sqlite, postgres, mysql } from "emdash/db";
+
+/**
+ * Database configuration from environment variables.
+ *
+ * Supports:
+ * - SQLite: DATABASE_URL=file:./storage/data.db
+ * - PostgreSQL: DATABASE_URL=postgresql://user:pass@host:5432/db
+ * - MySQL: DATABASE_URL=mysql://user:pass@host:3306/db
+ */
+export function getDatabaseConfig() {
+	const url = process.env.DATABASE_URL;
+
+	if (!url) {
+		// Default to SQLite
+		return sqlite({ url: "file:./storage/data.db" });
+	}
+
+	if (url.startsWith("file:")) {
+		// SQLite
+		return sqlite({ url: url });
+	}
+
+	if (url.startsWith("postgresql:")) {
+		// PostgreSQL
+		return postgres({ connectionString: url });
+	}
+
+	if (url.startsWith("mysql:")) {
+		// MySQL
+		return mysql({ connectionString: url });
+	}
+
+	// Fallback to SQLite
+	console.warn(`Unknown DATABASE_URL protocol, falling back to SQLite`);
+	return sqlite({ url: "file:./storage/data.db" });
+}