Make the draw a commit-reveal at a fixed, confirmed, quorum-agreed height with a public proof (B-73, RND-14 … RND-20, RND-33 … RND-37, DAT-61 … DAT-66, API-25, API-45, AUD-24 … AUD-27, AUD-37, ELC-15, SEC-56, ADR-24, ADR-25)

The seed was the hash of the first block seen after the close, so a miner holding tickets
could withhold blocks until one made them win, the active Electrum server could pick which
real block was used by not announcing another (a failed corroboration even moved the draw to
the next block), and the participant order was the server-only `broadcast_at` with no
tie-break, so nobody could recompute the result.

- Open: every round gets a 32-byte `server_seed`, stored only Fernet-encrypted with the
  master-xprv key (`server_seed_enc`), with its published `server_seed_hash`;
  `draw_confirmations` is copied from round_config. The plaintext column is written only by
  the reveal.
- Entering `drawing` (one transaction, `draw_committed` audit row): participants ordered by
  bet txid, each `draw_position`, `participants_hash` (sha256 of `txid:vout:address\n`
  lines; new `bet_vout`), `draw_height = tip_at_close + 1`. A tip of 0 keeps the round
  `closing` (`draw_baseline_tip_unknown`, once per round). All persisted, so a resumed draw
  reads rather than recomputes (the `_tick` branch itself is B-81).
- `_complete_draw` / `_wait_for_draw_block` replace `_wait_for_next_block`: wait for
  `draw_confirmations` blocks from `draw_height`, then `ElectrumListener.agreed_block_hash`
  asks every server, the active one included, for the header at that exact height (valid
  work, >= 2 other answers, strict majority of all); no agreement means the same height is
  asked again and `draw_header_corroboration_failed` is written once per distinct reported
  hash. `draw_stalled` now counts from the last tip advance, or from when the quorum started
  to fail. Seed = sha256(server_seed || block_hash || participants_hash), index = seed mod n.
- The winner, the seed and the reveal (`server_seed`, `winner_drawn` payload) are written in
  one transaction; a round closing with no participants reveals too. A round opened before
  this change (no commitment) is given a secret as it enters `drawing`.
- `GET /rounds/{id}/proof` (public, 404 `round_not_found`, translated in all 7 languages) and
  the new fields on `GET /rounds/current`; `draw_confirmations` in `/admin/config` and the
  /admin form; `server_seed_hash`/`draw_height` in `/admin/rounds`, never the secret.
- Alembic d3e8b15a7c42 (plain add_column / create_index, expression indexes untouched).
- Docs: the B-73 `*Stato:*`/`*Oggi:*` lines are gone, guides and CLAUDE.md describe the new
  draw. Ambiguities resolved in api.md: the proof's participants also carry `amount_sats`
  (RND-37 lists it); `seed` is decimal in the proof and hex in the audit row, as specified.

Tests changed because the spec changed: ElectrumListener.corroborate_header (asked only the
other servers about the block the active one announced) is replaced by agreed_block_hash and
its quorum tests ported; draw_winner takes the secret and the list fingerprint; the four
_wait_for_next_block scheduler tests are replaced by _wait_for_draw_block ones (fixed height,
confirmations, same-height retry, stall clock). New: test_draw_proof.py recomputes the winner
independently from the public proof, checks the plaintext secret is nowhere in the database,
the API or the log before the reveal, and that a second pass over a `drawing` round yields
the same winner. Mutation-checked: ordering by broadcast_at, an early reveal and asking the
tip instead of the draw height each fail tests.

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

tests/unit/test_migrations.py

@@ -160,3 +160,39 @@ def test_failed_at_and_paused_reason_are_backfilled_for_existing_rows(tmp_path):
     assert reason == "operator"
     assert _index_sql(db_path, "ix_rounds_single_active") is not None
     assert _index_sql(db_path, "ix_users_username_lower") is not None
+
+
+def test_the_commit_reveal_columns_and_the_draw_position_index_exist_at_head(migrated_db):
+    """B-73 (DAT-62 … DAT-66): the new columns, their defaults and the unique position index."""
+    with sqlite3.connect(migrated_db) as conn:
+        rounds = {r[1]: r for r in conn.execute("PRAGMA table_info(rounds)")}
+        for column in ("server_seed_enc", "server_seed", "server_seed_hash", "participants_hash", "draw_height"):
+            assert column in rounds and rounds[column][3] == 0  # nullable
+        assert rounds["draw_confirmations"][3] == 1 and rounds["draw_confirmations"][4] in ("3", "'3'")
+        participants = {r[1]: r for r in conn.execute("PRAGMA table_info(round_participants)")}
+        assert participants["bet_vout"][3] == 1 and participants["bet_vout"][4] in ("0", "'0'")
+        assert participants["draw_position"][3] == 0
+        config = {r[1]: r for r in conn.execute("PRAGMA table_info(round_config)")}
+        assert config["draw_confirmations"][4] in ("3", "'3'")
+
+    sql = _index_sql(migrated_db, "ux_round_participants_round_draw_position")
+    assert sql is not None and "unique" in sql and "(round_id, draw_position)" in sql
+
+
+def test_two_participants_cannot_share_a_draw_position_but_null_positions_coexist(migrated_db):
+    with sqlite3.connect(migrated_db) as conn:
+        conn.execute("INSERT INTO rounds (status, opened_at) VALUES ('closed', '2026-01-01 00:00:00')")
+        round_id = conn.execute("SELECT max(id) FROM rounds").fetchone()[0]
+        columns = "(round_id, user_id, bet_amount_sats, bet_txid, broadcast_at, status, draw_position)"
+
+        def insert(user_id, position):
+            conn.execute(
+                f"INSERT INTO round_participants {columns} VALUES (?, ?, 1, 'tx', '2026-01-01 00:00:00', 'confirmed', ?)",
+                (round_id, user_id, position),
+            )
+
+        insert(1, None)
+        insert(2, None)  # NULLs do not collide
+        insert(3, 0)
+        with pytest.raises(sqlite3.IntegrityError):
+            insert(4, 0)