Import source: backend/.env.example

Import the existing local project snapshot. This commit organizes existing files; it does not represent newly developed functionality.

backend/.env.example

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+PORT=4001
+NODE_ENV=development
+FRONTEND_URL=http://localhost:5173
+PUBLIC_APP_URL=http://localhost:5173
+DATA_DIR=./data
+MAX_FILE_SIZE_BYTES=524288000
+PREVIEW_MAX_FILE_SIZE_BYTES=25000000
+JWT_ACCESS_SECRET=replace-with-64-char-random-hex
+JWT_REFRESH_SECRET=replace-with-64-char-random-hex
+ADMIN_API_KEY=replace-with-64-char-random-hex
+BCRYPT_ROUNDS=12
+
+# Future production adapters from CONTINUE.md
+DATABASE_URL=postgresql://user:pass@localhost:5432/cshare
+REDIS_URL=redis://localhost:6379
+STORAGE_ENDPOINT=https://<account>.r2.cloudflarestorage.com
+STORAGE_BUCKET=cshare-files
+STORAGE_ACCESS_KEY=
+STORAGE_SECRET_KEY=
+STORAGE_PUBLIC_URL=https://files.example.com
+RESEND_API_KEY=