Meet BIP125's relay minimum on every RBF bump, and cap the fee rate (B-32)

bump_fee computed fee_delta as new_fee - old_fee, falling back to a
flat 1-satoshi bump whenever that came out zero or negative - which
happened whenever old_fee (the actual fee paid, from real prevout
amounts) already exceeded the naive target, e.g. because dust change
had been folded into the original fee (psbt_builder.py's
DUST_LIMIT_SATS handling). A 1-satoshi total increase is nowhere near
BIP125 rule 4's minimum (the replacement must pay at least the
incremental relay fee rate times its own vsize more than what it
replaces), so the node rejected it every time - and since bump_fee
raised before touching `pending`, the next tick retried with identical
parameters every 30 seconds, forever. Separately, the fee rate climbed
by 1 sat/vB every bump with no ceiling.

fee_delta is now max(target_fee - old_fee, vsize * the incremental
relay rate) - always at least the relay-mandated minimum regardless of
what the naive arithmetic produces. pending.fee_rate_sat_vb is set to
the actual resulting rate rather than the naive target, so a later
bump's arithmetic starts from what's really being paid instead of
drifting from it. Once a transaction reaches MAX_FEE_RATE_SAT_VB (a
new constant, 10,000 sat/vB, shared with RoundConfig.fee_rate_sat_vb's
existing admin-facing bound so the two can't drift apart - the same
reason MIN_PASSWORD_LENGTH is shared elsewhere) bump_fee refuses to
bump further; the reconciler abandons it if it never confirms (B-27)
instead of this retrying forever.

Suite grows from 185 to 187 tests. BUGS.md moves B-32 to Previously
fixed.

app/api/routes/admin.py

@@ -14,6 +14,7 @@ from app.db.session import get_session
 from app.rounds.config import get_round_config
 from app.wallet.address import is_valid_plm_address
 from app.wallet.hd import derive_user_wif
+from app.wallet.psbt_builder import MAX_FEE_RATE_SAT_VB
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 
@@ -68,7 +69,7 @@ class RoundConfigUpdate(BaseModel):
     bet_amount_sats: int | None = Field(default=None, gt=0, le=100_000 * 100_000_000)
     round_duration_seconds: int | None = Field(default=None, ge=30, le=7 * 24 * 3600)
     round_cooldown_seconds: int | None = Field(default=None, ge=0, le=24 * 3600)
-    fee_rate_sat_vb: int | None = Field(default=None, ge=1, le=10_000)
+    fee_rate_sat_vb: int | None = Field(default=None, ge=1, le=MAX_FEE_RATE_SAT_VB)
     rbf_timeout_seconds: int | None = Field(default=None, ge=60, le=7 * 24 * 3600)
     draw_animation_seconds: int | None = Field(default=None, ge=0, le=600)