Ban-pick: one-time pool veto for low-rated players (port of Botlatro's tuple veto)

### What
Port Botlatro-Multiplayer's match **veto** into the in-game ban-pick draft: a
one-time-per-match button letting a low-rated player reroll the candidate pool,
with a difficulty-mercy bias.

### Reference semantics (Botlatro-Multiplayer, verified in source)
- Tuple-ban queues generate 9 weighted (deck, stake) tuples; teams ban from the list.
- **Veto** (`veto-tuples-` handler): only match participants with
`elo <= queues.veto_mmr_threshold` (fallback 200) may press it. Effect: full
tuple-list regeneration with a mercy rule -- `TupleBans.veto()` forces white-stake
generation when the list holds fewer than 4 white-stake options
(`vetoWhiteAmount = 4`). **One veto per match, shared** (`tupleVetoUsed` keyed by
match id): if both players are eligible, first press consumes it. No vote --
unilateral, by design (it is a low-rated-player protection).
- Contrast: **Reroll Options** is the consensual sibling -- any participant may
trigger it but it runs a vote of all match players, also once per match.
- Rank names (STONE etc.) are Discord roles (`queue_roles.mmr_threshold`) and are
**display-only**; the veto gate is an independent raw threshold. The bot does not
tie veto to rank either -- the numbers are just aligned by convention.

### Two bot bugs the port should NOT copy
1. Button visibility checks the WHOLE QUEUE, not the match:
`SELECT elo FROM queue_users WHERE queue_id = $1` + `.some(...)`
(matchHelpers.ts) -- the VETO button renders whenever anyone in the queue is
under threshold, and the real gate only happens at press time.
2. Open TODO: the veto button disappears permanently after a reroll.

### Design: server-computed eligibility (client never knows the rule)
The server computes `can_veto` per player per match and includes it in the
match/draft payload. The client renders the button only when true; the host/server
still validates the veto action itself (client-side gating alone is spoofable).

This keeps the eligibility RULE server-side and swappable: start with bot parity
(rating <= threshold in gamemode/queue config, using `matchmaking_ratings.rating`),
and later move to named tiers if/when the server grows a tier concept -- today the
schema has only integer ratings and leaderboard positions, no named ranks -- all
without touching the mod or API.

### Depends on
#13 (weighted tuple pool policy) -- the mercy rule only has meaning against a weighted tuple pool. Eligibility and pool policy are both per `modId + gameMode`, so PvP and Speedrun configure independently.

### Work items
1. **Server:** veto threshold in gamemode/queue config; compute per-player
`can_veto` into the match payload; validate + consume the veto action
(once per match) server-side.
2. **API engine:** `veto` action -- host regenerates the pool (consumer
`build_pool` re-run with a mercy flag), resets the ban schedule, broadcasts;
once-per-draft guard mirrored in host state.
3. **API UI:** Veto button in the ban-pick button row (definition-time `button`
config + per-frame check, same pattern as Confirm/Random); rendered only when
the payload says `can_veto`; removed once consumed.
4. **Speedrun mod:** pool builder honours the mercy rule (the white-stake-bias
equivalent for its pools; engine already supports `{key, stake}` tuple items +
`decorate_tile`).

### Open questions
- Threshold value per gamemode (bot default is 200; new-server ratings default 600
-- numbers need re-basing).
- Veto on plain deck drafts too, or only tuple (deck+stake) pools?
- Repool semantics: bot restarts the tuple ban list -- mirror that (full draft
restart) or preserve prior bans?

Start with the gamemode/queue config, matchmaking_ratings.rating, and the match/draft payload. Trace the API engine's build_pool flow, host state and Confirm/Random button configuration, then inspect decorate_tile in the Speedrun mod. Done means server-computed can_veto, server-side one-time consumption and broadcast, a correctly gated UI button, and mercy-aware pool generation.

Written by the indexing model from the issue text.