The image tag prefixed `ig-` when `..._meta_mode` was `instagram`, but this role
does not accept that mode any more: `..._database_suffix` and
`..._bridge_username_prefix` map only `facebook`, `facebook-tor` and `messenger`,
so `instagram` raises an undefined-key error long before the tag is rendered.

Instagram has been a separate bridge since v26.07 and is handled by
matrix-bridge-mautrix-meta-instagram, which hardcodes the prefix. Left over from
before that split.

The comment stays, reworded: both bridges still share a container image
repository, so an `ig-` prefix on a tag here would mean this role had pulled the
other bridge's image - which is what the Molecule scenario asserts against.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT