Make usernames one case-insensitive namespace (B-57)

The login throttle keyed on body.username.lower() while the lookup matched
User.username exactly, so "Bob" and "bob" were two accounts sharing one
rate-limit bucket — each able to lock the other out — and registration happily
accepted near-duplicate names, which on a custodial system is an impersonation
vector.

Uniqueness is now the database's job: a unique index on lower(username), with
register and login both matching through func.lower(). The name is still stored
exactly as typed, since that's what /admin and the audit log display, and the
username pattern is ASCII-only so lower() is the whole of the normalization.

The migration refuses to run if two existing accounts differ only by case. It
can't merge or rename one automatically: both are custodial accounts that may
hold funds, so that would be the migration silently deciding who owns what. It
names the collisions and leaves them to the operator — the container runs
`alembic upgrade head` at startup, so it surfaces as a refusal to start rather
than a half-applied schema. Verified both directions against a scratch DB, plus
`alembic check` (clean) and the collision guard actually firing.

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

CLAUDE.md

@@ -8,7 +8,7 @@ The user communicates in Italian in chat — reply to them in Italian. Everythin
 
 ## Project status
 
-All 10 stages of the original build order are code-complete and unit-tested — 287 tests, all under `tests/unit/` (`tests/integration/` is an empty package). Beyond them: Docker + Caddy deployment, admin dashboard (`/admin`), static test UI (`/`), pending-inclusive balance display, an SSE push channel layered over the original polling, self-service password change + admin password reset, and the reconciliation/corroboration machinery below.
+All 10 stages of the original build order are code-complete and unit-tested — 290 tests, all under `tests/unit/` (`tests/integration/` is an empty package). Beyond them: Docker + Caddy deployment, admin dashboard (`/admin`), static test UI (`/`), pending-inclusive balance display, an SSE push channel layered over the original polling, self-service password change + admin password reset, and the reconciliation/corroboration machinery below.
 
 Verified on mainnet with real money: registration + address derivation, deposit crediting (1-conf), a real 10 PLM bet (broadcast → confirmed → change credited back), and one full round cycle (close → draw on a real block hash → 70/30 payout with sat math checked against the broadcast tx → confirmation → close → next round auto-opened). **Withdrawal and the RBF bump path have never been exercised against a live broadcast** — unit-tested only.
 
@@ -33,7 +33,7 @@ PYTHONPATH=. python scripts/decrypt_master_key.py     # ops recovery: decrypt+pr
 PYTHONPATH=. python scripts/encrypt_master_key.py     # ops bootstrap: import an externally-generated xprv (--overwrite to replace)
 PYTHONPATH=. python scripts/electrum_smoke_test.py    # manual check: connect, handshake, subscribe to headers, print the tip
 
-python -m pytest                                      # all 287 tests
+python -m pytest                                      # all 290 tests
 python -m pytest tests/unit/test_hd.py                # one file
 python -m pytest tests/unit/test_hd.py::test_derivation_is_deterministic   # one test
 ```
@@ -232,6 +232,7 @@ Explicit design choices, not derivable from any single file — respect them:
 - **1 confirmation** for every tx kind. Don't introduce differing thresholds (3, 6, …) without an explicit decision.
 - The draw algorithm is a **replaceable component**, not the final design — don't architect around its current form.
 - `GET /admin/users/{id}/privkey` exporting a raw WIF is **intentional**, not a vulnerability: the server already holds the master key, so this only exposes via API what an operator could script anyway. Every access writes `admin_privkey_accessed` — don't remove that logging.
+- **Usernames are case-insensitive** (B-57): one namespace, enforced by a unique index on `lower(username)` (`app/db/models.py`) and matched with `func.lower(...)` on both register and login. The name is still *stored* as typed — that's what `/admin` and the audit log show. The migration refuses to run if two existing accounts differ only by case, rather than guessing which one to rename: both may hold funds.
 - Argon2 hashing means **no password recovery, only reset**: `POST /admin/users/{id}/reset-password` sets a new random password, returns it once for the operator to relay, and logs `admin_password_reset`. No self-service reset exists (no email is ever collected); a logged-in user can only *change* their password by supplying the current one.
 - RBF bumps are paid by whoever's change the tx pays back to — the user for bets/withdrawals, the pool for payouts. Counterparty outputs (recipient, winner, fee address) are never touched; only the sender's own change shrinks (`bump_fee`).