Fix: Make CI .env manipulation robust against missing trailing newlines (#15189)

## Problem

CI workflow started timing out on October 14, 2025 after commit
`d750df7fff` removed the trailing newline from `.env.example`.

## Root Cause

When `.env.example` lacks a trailing newline:
```bash
# Last line without newline
# CLICKHOUSE_URL=...twenty
```

And CI runs:
```bash
echo "NODE_PORT=3002" >> .env
```

Result:
```bash
# CLICKHOUSE_URL=...twentyNODE_PORT=3002  ← Commented out!
```

Server starts on default port 3000 instead of 3002, health check fails.

## Fix

1. **Restore trailing newline** to `.env.example`
2. **Make all CI `.env` operations robust** by adding `echo "" >> .env`
before appending
3. **Simplified `set_env_var`** function to always add newline first

Now works regardless of whether template files have trailing newlines.

## Files Changed

- 6 CI workflow files
- 1 .env.example file

.github/workflows/preview-env-keepalive.yaml

@@ -37,6 +37,7 @@ jobs:
           cp packages/twenty-docker/.env.example packages/twenty-docker/.env
           
           echo "Generating secrets..."
+          echo "" >> packages/twenty-docker/.env
           echo "# === Randomly generated secrets ===" >> packages/twenty-docker/.env
           echo "APP_SECRET=$(openssl rand -base64 32)" >> packages/twenty-docker/.env
           echo "PG_DATABASE_PASSWORD=$(openssl rand -hex 16)" >> packages/twenty-docker/.env
@@ -60,6 +61,7 @@ jobs:
           # Update the SERVER_URL with the tunnel URL
           echo "Setting SERVER_URL to ${{ steps.expose-tunnel.outputs.tunnel-url }}"
           sed -i '/SERVER_URL=/d' .env
+          echo "" >> .env
           echo "SERVER_URL=${{ steps.expose-tunnel.outputs.tunnel-url }}" >> .env
           
           # Start the services