Implement full MVP: auth, HD wallet, Electrum client, deposits, bets, round/draw engine, payout, withdrawals, RBF, admin+audit

All 10 build-order stages complete and unit-tested (49 tests). Verified live on
mainnet: registration/address derivation, deposit crediting, a real 10 PLM bet
(broadcast + confirmed + change credited). A full round close->draw->payout
cycle was triggered live and was in progress at commit time. Withdrawal and RBF
bump are unit-tested but not yet exercised against a live broadcast. Known gaps
(scheduler doesn't resume mid-flight rounds after restart, payout has no retry,
no deployment setup, etc.) are documented in CLAUDE.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/rounds/draw.py

@@ -0,0 +1,21 @@
+import hashlib
+
+
+def header_hex_to_block_hash(header_hex: str) -> str:
+    """Block hash from a raw Electrum header: sha256d, byte-reversed, hex.
+    Verified against a real mainnet block (blockchain.transaction.get's own
+    reported blockhash) during development."""
+    header_bytes = bytes.fromhex(header_hex)
+    digest = hashlib.sha256(hashlib.sha256(header_bytes).digest()).digest()
+    return digest[::-1].hex()
+
+
+def draw_winner(participants: list[str], block_hash_hex: str) -> str:
+    """v1 draw algorithm (flowchart.mmd, node R): seed = block hash as an integer,
+    index = seed mod participant_count, winner = participants[index]. Anyone can
+    recompute and verify it from public data. Deliberately simple/replaceable."""
+    if not participants:
+        raise ValueError("no participants to draw from")
+    seed = int(block_hash_hex, 16)
+    index = seed % len(participants)
+    return participants[index]