Resend activation outreach

Re-enqueue activation outreach for a pending (or active) member.

Idempotent: if a pending or sent row already exists for a channel, no duplicate is created (the outbox idempotency backstop applies). Deactivated members are rejected, because a removed member cannot receive an activation nudge.

Once-ever: the welcome is one hello per membership across BOTH channels, not one per channel. Whichever lane says it first spends the claim (the DM lane before it posts, the email lane once the provider accepts), so a member whose claim is already spent gets nothing on either channel. This refuses with skipped_reason="member_already_welcomed" rather than minting a row the drainer would terminally skip moments later.

Nothing is minted until somebody in the workspace has given Patricia a task in chat, so an empty channels can carry skipped_reason="workspace_has_not_used_patricia". The workspace outreach setting plays no part: it allows or stops marketing email only.

Attempt-bounded: a member whose welcome DM has already failed its allowance is refused with skipped_reason="welcome_dm_attempts_exhausted" rather than queueing another attempt the platform has already refused three times.

A created email row is delivered by its own background task. A created DM row (slack_dm or teams_dm) is NOT: it is picked up by the paced welcome DM drainer, so "created" here means queued for delivery within the next few minutes, not sent.

Returns the channels for which new outreach rows were created. Empty channels means the workspace has not used Patricia yet, the member has already been welcomed (see skipped_reason for both), or the applicable channel already had a pending/sent row.