Give every payout a bump reserve output and refuse configs that cannot pay a round (B-79, RND-21, RND-23, RND-24, RND-27, RND-38, RND-40, DAT-73, API-31, API-36, TX-11, ADR-17, ADR-29)

The pool normally holds exactly the round's bet outputs and the payout spent them all for
exactly the pool amount, so it had no change output: bump_fee could never accelerate it,
and a payout stuck in a mempool (never abandoned while a server knows it) kept the round in
`paying_out` and the whole lottery stopped.

- build_payout_transaction always has three outputs: winner, fee address, and a bump reserve
  of max(DUST_LIMIT_SATS, estimate_vsize(inputs, 3) * 10 sat/vB) paid to the reserve address
  m/84'/746'/0'/1/0. The reserve comes out of the platform's 30%, the network fee still out
  of the winner's 70%. Input value beyond winner share + commission (only rounds opened
  before the per-round pool address) is folded into the reserve output if >= dust, left to
  the network fee otherwise. `change_address` is replaced by `reserve_address`.
- `rounds.bump_reserve_sats` (Alembic e5a91c3d7b26, plain add_column) is written with the
  payout intent before the broadcast; /admin/rounds returns it.
- bump_fee takes the increase from the reserve output for a payout
  (_find_reducible_output; a payout already in flight with a change to the pool still falls
  back to that), never from the winner or fee-address outputs.
- payout_config_problem / rounds_can_open (RND-40): a bet amount and fee rate under which a
  round of 1 or of MAX_PARTICIPANTS_PER_ROUND participants could not cover the network fee,
  the reserve and dust outputs opens no round (lottery_configured=false); PUT /admin/config
  refuses such a bet amount or fee rate with 422 config_not_payable, judged on the resulting
  values, since the payout reads the fee rate live.

Ambiguity resolved: the reserve output and any excess input are one output (RND-23 says one
output to the reserve address), and bump_reserve_sats records that output's initial value.
DAT-22's "abandon clears all amounts" stays with B-96.

Tests changed because the spec changed: the four test_payout_builder tests asserted the old
two/three-output shape with change to the pool and the fee address receiving the whole 30%;
the whitespace fee_address test now spells out the amounts rounds_can_open reads. New: reserve
sizing, payout with 1 and with 400 participants (inputs = outputs + fee to the satoshi),
exact commission/reserve/dust boundary, excess folded or left to the fee, a real payout bumped
out of its reserve with the other outputs byte-identical and bumped five times, the old
reserve-less shape still raising RbfError, the scheduler output addresses and recorded
amounts, payability boundaries found independently by bisection, no round / unconfigured flag
for an unpayable config, a round in progress surviving it, PUT /admin/config refusals and the
migration column. Mutation-checked: dropping the reserve lookup in bump_fee and the payability
check in rounds_can_open each fail tests.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/adr/ADR-17-commissione-pagamento-esce-dalla-quota-vincitore.md

@@ -1,6 +1,6 @@
 # ADR-17 — La commissione del pagamento esce dalla quota del vincitore; la riserva per l'RBF esce dalla quota della piattaforma
 
-- **Stato:** Accettata — implementazione da completare (B-79)
+- **Stato:** Accettata
 - **Area:** Economia del gioco
 - **Fonte di verità per:** la scelta di progetto descritta qui sotto; i requisiti (`PREFISSO-NN`) che la attuano stanno nei documenti di dominio, indicati nelle *Conseguenze*.
 
@@ -30,6 +30,4 @@ commissione verso `fee_address` è quindi il 30% meno la riserva.
   [ciclo-di-vita-del-round.md](../ciclo-di-vita-del-round.md#4--pagamento) (B-79).
 
 *Codice:* `app/wallet/psbt_builder.py:build_payout_transaction` ·
-*Test:* `tests/unit/test_payout_builder.py::test_payout_deducts_fee_only_from_winner_share`
-*Stato:* da implementare (B-79) · *Oggi:* nessuna riserva: il 30% va intero a
-`fee_address` e, nel caso normale, il pagamento non ha resto.
+*Test:* `tests/unit/test_payout_builder.py::test_payout_deducts_fee_only_from_winner_share`, `tests/unit/test_payout_builder.py::test_payout_at_the_participant_cap_pays_the_reserve_out_of_the_platform_share`, `tests/unit/test_broadcast.py::test_a_payout_is_bumped_out_of_its_reserve_output_and_nothing_else`

docs/adr/ADR-29-senza-fee-address-apre-nessun-round.md

@@ -1,6 +1,6 @@
 # ADR-29 — Senza `fee_address` non si apre nessun round
 
-- **Stato:** Accettata — implementazione da completare (B-79)
+- **Stato:** Accettata
 - **Area:** Round ed estrazione
 - **Fonte di verità per:** la scelta di progetto descritta qui sotto; i requisiti (`PREFISSO-NN`) che la attuano stanno nei documenti di dominio, indicati nelle *Conseguenze*.
 
@@ -19,6 +19,4 @@ round impagabile va aggiunta nello stesso punto.
   l'RBF e una commissione sopra la soglia di polvere ([ADR-17](ADR-17-commissione-pagamento-esce-dalla-quota-vincitore.md), B-79).
 
 *Codice:* `app/rounds/service.py:rounds_can_open` · `app/rounds/config.py:get_round_config` ·
-*Test:* `tests/unit/test_rounds_service.py::test_withholds_new_round_while_no_fee_address_is_configured`, `tests/unit/test_rounds_service.py::test_a_round_in_progress_survives_the_fee_address_being_cleared`, `tests/unit/test_rounds_route.py::test_lottery_configured_flags_a_missing_fee_address`
-*Stato:* da implementare (B-79) · *Oggi:* `rounds_can_open` controlla solo
-`fee_address`.
+*Test:* `tests/unit/test_rounds_service.py::test_withholds_new_round_while_no_fee_address_is_configured`, `tests/unit/test_rounds_service.py::test_a_round_in_progress_survives_the_fee_address_being_cleared`, `tests/unit/test_rounds_route.py::test_lottery_configured_flags_a_missing_fee_address`, `tests/unit/test_rounds_service.py::test_no_round_opens_while_the_config_could_not_pay_one`, `tests/unit/test_rounds_route.py::test_lottery_configured_is_false_when_the_amounts_could_not_pay_a_round`