I need you to create a new FastAPI endpoint called POST /api/lia/outbound.

Context:
- This endpoint will be called by Make.com to generate outbound WhatsApp messages for renewal customers.
- We already have inbound logic at /api/lia/inbound and campaign rules inside lia_rules.py (EOT_12, EOT_9, EOT_6, EOT_3 selection).
- We want outbound to use that same campaign logic automatically.

Please create a new FastAPI router file if needed (for example app/routers/lia_outbound.py), or add to the existing LIA router if that is how the project is structured.

Requirements:

1. Create a Pydantic model:
    class OutboundRequest(BaseModel):
        customer_id: Optional[str]
        reg: Optional[str]
        payments_remaining: Optional[int]
        campaign: str = "pcp_stage_auto"
        channel: str = "whatsapp"
        sandbox_mode: bool = False

2. Create an OutboundResponse model:
    class OutboundResponse(BaseModel):
        message_text: str
        conversation_id: Optional[str] = None
        to_number: Optional[str] = None
        stage: Optional[str] = None

3. Implement POST /api/lia/outbound using this logic:

    - If campaign == "pcp_stage_auto":
        Use payments_remaining to automatically select the correct EOT stage via existing logic in lia_rules.select_eot_intro() or the equivalent function.
    - Generate the outbound message text using the correct template.
    - Assign a "conversation_id" (you can reuse customer_id for now or generate a UUID).
    - For sandbox_mode=True:
         * Do NOT use the real customer number; return None.
         * This is because Twilio sandbox only works with pre-approved numbers.
    - Return OutboundResponse.

4. Include the router in main.py or app/__init__.py using:
        app.include_router(lia_outbound.router)

5. Make sure the new route appears in /docs.

The endpoint does NOT need to send the WhatsApp message. It only prepares the outbound content. Make will send the message via Twilio.

Once complete:
- Tell me the file(s) created or modified.
- Confirm the endpoint is available at /api/lia/outbound.