We now want /api/lia/outbound to populate the to_number field using dp_customer.phone_mobile, so that Make/Twilio can use the API response directly.

Context:
- dp_customer has column phone_mobile (e.g. '+447972194025').
- OutboundResponse currently returns:
  - message_text
  - conversation_id
  - to_number (often null)
  - stage
- /api/lia/outbound already receives a valid customer_id (UUID).

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TASK – Populate to_number from dp_customer.phone_mobile

1. Open app/lia_outbound.py and locate the place where OutboundResponse is created in process_outbound_message (or equivalent). It currently looks like:

    return OutboundResponse(
        message_text=message_text,
        conversation_id=str(conversation_id),
        to_number=to_number,   # often None
        stage=stage,
    )

2. Before building the OutboundResponse, add logic to:

   - Look up the customer from dp_customer by payload.customer_id.
   - If a row exists and phone_mobile is not null/empty:
       - Build to_number as: "whatsapp:" + phone_mobile
         e.g. phone_mobile = '+447972194025'
              to_number    = 'whatsapp:+447972194025'
   - If no row is found OR phone_mobile is null/empty:
       - Raise an HTTP 400 error with a clear message, for example:
         HTTPException(
             status_code=400,
             detail="No mobile phone number found for this customer_id in dp_customer"
         )

3. Apply this behaviour for BOTH sandbox_mode true and false:
   - sandbox_mode continues to control whether this is a “test” run,
   - but if a phone number exists, to_number should still be included in the JSON response so Make/Twilio can be wired consistently in test and production.

4. Do NOT change:
   - How message_text is generated,
   - How conversation_id or stage are derived,
   - Any LLM/URE logic.

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TEST

After the change, call /api/lia/outbound with this payload:

```json
{
  "customer_id": "44f01301-7a59-4e34-b0a8-d0d89e061aab",
  "payments_remaining": 12,
  "campaign": "TEST_12_PCP",
  "asset_type": "new",
  "channel": "whatsapp",
  "sandbox_mode": true,
  "from_number": "whatsapp:+44YOUR_SANDBOX_OR_BUSINESS_NUMBER"
}