fix: emit absolute approval URLs from messenger push scripts

[retog]

Problem

When an outbound messenger send is queued for approval (verify policy), the push scripts (whatsapp-push.py, signal-push.py, telegram-push.py) print an approval link. The gateways build that link from SEND_APPROVAL_BASE_URL only — when that variable is unset, they return a bare relative path (/sends/<account>/<id>). The scripts printed it verbatim, so the host had to be prepended downstream by hand, which repeatedly produced the wrong host.

Fix

Absolutize the relative approval path against CONVERSATION_BASE_URL, which is already present in the container that runs the push scripts. This mirrors the existing, correct fallback in email_client.approval_url():

base = (os.environ.get("SEND_APPROVAL_BASE_URL")
        or os.environ.get("CONVERSATION_BASE_URL", "")).rstrip("/")

After this change the printed link is always complete and copy-ready, on every channel — the host can no longer be assembled (and mis-assembled) by hand.

Safety

• No regression when CONVERSATION_BASE_URL is unset: falls back to the relative path (today's behaviour).
• No change when the gateway already returns an absolute URL (path does not start with /, so it is left untouched).

Follow-up after merge

python3 /workspace/scripts/self-update.py to roll the change into the running stack.

[aros-agent]

Written by Aros, the project's AI agent, from my own account @aros-agent.

Reviewed at head 0e46686. The fix is correct and it sits at the right layer: the three gateway services have no env_file in docker-compose.yml, so they can never see CONVERSATION_BASE_URL — only the container running the push scripts can (it gets the whole .env via env_file). Verified against source rather than the PR body: signal-gateway.py:1754 does return the bare relative path when SEND_APPROVAL_BASE_URL is unset, import os is present in all three scripts, and startswith("/") correctly leaves an already-absolute URL untouched, so there is no double-prefix case.

One consistency note, not a defect in the shipped wiring:

The in-code comment says the block "mirrors email_client.approval_url()", but the mirror is partial. email_client.py:999 is

base = (os.environ.get("SEND_APPROVAL_BASE_URL")
        or os.environ.get("CONVERSATION_BASE_URL", "")).rstrip("/")

while the new block reads only CONVERSATION_BASE_URL. In the shipped compose this never bites — a SEND_APPROVAL_BASE_URL set in .env reaches the gateways, which then return absolute URLs and the fallback is inert. The narrow window is a deployment where the push-script container has SEND_APPROVAL_BASE_URL but the gateway process doesn't (e.g. the variable was added to .env and the gateway container wasn't recreated): the link then stays relative even though a usable base is one os.environ.get away. Using the exact two-variable expression from email_client costs one line per script and makes the comment literally true.

Fine to merge as-is; the note is cheap to fold in now or ignore.