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>

tests/unit/test_rounds_route.py

@@ -296,3 +296,23 @@ async def test_current_round_exposes_its_own_pool_address(client):
 
     resp = await ac.get("/rounds/current")
     assert resp.json()["pool_address"] == derive_pool_address(5)
+
+
+async def test_lottery_configured_is_false_when_the_amounts_could_not_pay_a_round(client):  # B-79 (RND-40)
+    from sqlalchemy import select
+
+    from app.db.models import RoundConfig
+
+    ac, session_factory = client
+    async with session_factory() as session:
+        session.add(
+            RoundConfig(fee_address="plm1q5x25wd6q463mfhckjraaedgjg0lyu73qfcj43n", bet_amount_sats=5_000)
+        )
+        await session.commit()
+    body = (await ac.get("/rounds/current")).json()
+    assert body["lottery_configured"] is False and body["round_id"] is None
+
+    async with session_factory() as session:
+        (await session.scalars(select(RoundConfig))).one().bet_amount_sats = 1_000_000_000
+        await session.commit()
+    assert (await ac.get("/rounds/current")).json()["lottery_configured"] is True