#15 · Show bot's own scoreboard rank in `!score` output

Context

!score already returns a member's points. Add a one-liner showing the bot's leaderboard rank within the community to drive engagement ("You're #14 of 312, 280 points behind #13").

Acceptance criteria

• [ ] !score adds a line: Rank: #N of M (X pts behind next, Y pts ahead of previous)
• [ ] Edge cases handled: rank 1 (no "behind"), rank last (no "ahead")
• [ ] Member opted out of leaderboard (leaderboard_visible=0): show only own score, no rank

Technical hints

• scoreboard exposes /api/leaderboard (already exists) — extend the response or add ?around_user=<id> if needed
• Modify handleScore in selfhost/messaging-apps/signal-messenger/src/src/bot/command-handler.ts (the switch dispatches !score/!leaderboard there)

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