Add player kicking for admins

Admins can now kick players from the Admin panel so a vanished player can't
stall the table. crud.kick_player removes a player mid-game: it drops their
captaincy, deletes their votes and any Challenge they're part of, and repairs
every other player's pointers to them (pending swap offers are cleared;
delegated Like/Hate questions and recruit technique slots are handed to a
present crewmate, or pool-filled when no one is left). The row is then deleted
— a kicked player's hand simply stops being "in play", so it returns to the
discard automatically. Finally recheck_phase_completion advances the phase if
the kicked player was the last thing it was waiting on.

Auth mirrors set-admin (admin-key on POST /api/game/{gid}/admin/kick). The one
guard is that the last remaining Admin can't be kicked, so the table is never
locked out of its own panel; a vanished creator can still be kicked once a
co-admin exists. The Admin panel gains a per-row Kick button (with a confirm),
shown as "—" for the last admin.

To support re-checking phase completion after a kick, each phase's
"everyone's done -> advance" gate was extracted into an idempotent maybe_*
helper, and recheck_phase_completion dispatches the right one per Game.phase.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TODO.md

@@ -1,8 +1,6 @@
 ## Major features
 - [ ] **Rollback of game state.** This may require some updates to how game state is handled in the backend (my understanding is that the frontend always gets full state updates rather than deltas, so it should be unaffected). Feel free to propose significant overhauls/refactors to enable this cleanly, I am not attached to the current database architecture at all. Also feel free to ask clarifying questions before going ahead with implementation. I think a clean UI for this would involve the event log. Each event gets a "rollback" button to roll back the state of the game to a checkpoint just after that event was posted. Until a new event is posted, the later entries in the log remain in a greyed out state so that the rollback can be undone if it was accidental. These buttons appear to the admin player if dev mode is on, and also to Deep players, but only going back as far as the start of the current scene if they aren't the admin.
 
-- [ ] **Player kicking.** Admins should be able to kick players out of the game so that it doesn't get stuck if a player vanishes. Admins can kick other admins. A kicked player's cards go to the discard.
-
 ## Words Words Words
 
 - [ ] **Example Play formatting.** The Example Play section of the Rules page could use a bit more formatting for legibility. Make diagrams of the card state at each step rather than just a text listing.