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>

.env.example

@@ -0,0 +1,36 @@
+# EmDash Environment Variables
+# Copy this file to .env and fill in your values
+
+# ===========================================
+# DATABASE (choose one)
+# ===========================================
+
+# SQLite (default - local file)
+# DATABASE_URL=file:./storage/data.db
+
+# PostgreSQL
+# DATABASE_URL=postgresql://user:password@host:5432/emdash
+
+# MySQL
+# DATABASE_URL=mysql://user:password@host:3306/emdash
+
+# ===========================================
+# SESSION SECRET (required for production)
+# ===========================================
+# Generate with: openssl rand -base64 32
+# SESSION_SECRET=your-secret-key-here
+
+# ===========================================
+# S3 STORAGE (optional - for cloud storage)
+# ===========================================
+# AWS S3
+# S3_BUCKET=my-uploads
+# S3_REGION=us-east-1
+# AWS_ACCESS_KEY_ID=xxx
+# AWS_SECRET_ACCESS_KEY=xxx
+
+# Cloudflare R2
+# R2_ACCOUNT_ID=xxx
+# R2_ACCESS_KEY_ID=xxx
+# R2_SECRET_ACCESS_KEY=xxx
+# R2_BUCKET=my-uploads