fix(signal): react in permitted groups regardless of DM allowlist

[pacewicz]

Problem

_reactions_enabled (gateway/platforms/signal.py) gates the 👀/✅ reactions only on the DM allowlist (SIGNAL_ALLOWED_USERS). A group message has already passed the SIGNAL_GROUP_ALLOWED_USERS intake gate before this fires, so it is already trusted — yet blanking SIGNAL_ALLOWED_USERS to deny stranger DMs also silenced the reactions inside permitted groups.

Fix

React when a group message's bare group id (chat_id_alt) is in SIGNAL_GROUP_ALLOWED_USERS, regardless of the DM allowlist.

Scope note

Narrowed to the reaction gate only. The parallel authz change (honoring SIGNAL_GROUP_ALLOWED_USERS in _is_user_authorized) is already covered by the existing cluster — #58187 / #52240 / #44706 — so it's dropped here to avoid duplication (per the triage note).

Testing

pytest signal / reaction suites green.

[alt-glitch]

This was generated by AI during triage.

Related to #52240 and #53348 in the Signal group-authz cluster. This PR also covers the separate reaction gate; please consolidate the overlapping authorization changes before merge.

[pacewicz]

Thanks — good catch. Narrowed this PR to just the reaction gate (_reactions_enabled), which the cluster doesn't cover (grep of #58187 for _reactions_enabled = 0). Dropped the authz commit since it duplicates #58187 / #52240 / #44706 — #58187's one-liner is the cleanest for the authz side, happy to see that land there.