Bound registrations with a per-IP quota instead of failure backoff (B-58)

The registration throttle called record_failure on every attempt, successful
ones included. Five legitimate signups from one shared or NAT address locked the
sixth real user out for up to 600s, doubling from there — while an attacker
sidestepped the limiter entirely through B-54. Failure backoff is the wrong
instrument here: nothing about creating an account is a failed guess at a
secret, so the only people it reliably punished were the honest ones.

RollingQuota says what was actually meant: 5 accounts per IP per hour, in a
rolling window. The caller over it waits exactly until the oldest of the five
ages out — an accurate Retry-After, and waiting never makes the next wait
longer. It is recorded only once an account exists, so attempts that create
nothing (a taken username, a validation error) leave the quota untouched, and
checked before the Argon2 hash, so an IP out of quota costs nothing to refuse.

Bounded like the failure limiter (B-56): the keys are caller-chosen, so the dict
gets both a sweep and a hard cap, evicting keys with room left in their quota
before full ones.

Also fixes the inline comment that cited B-31 (the resubscribe finding) where it
meant B-33.

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

app/auth/rate_limit.py

@@ -110,6 +110,84 @@ class RateLimiter:
         self._buckets.pop(key, None)
 
 
+class RollingQuota:
+    """How many times a key may do something in a rolling window — as opposed to
+    RateLimiter above, which punishes *failures* with a growing delay.
+
+    B-58: registration was throttled with the failure limiter, and recorded a
+    "failure" on every attempt, successful ones included. Five legitimate signups
+    from one shared or NAT address locked the sixth real user out for up to 600s,
+    with the backoff doubling from there — while an attacker sidestepped the whole
+    thing through B-54. The intent (bound how many accounts one source can create)
+    is right; failure backoff is the wrong instrument for it, since nothing here is
+    a failed guess at a secret. A quota says exactly what is meant: this many
+    accounts per source per window, and the answer to the one over it is "not yet",
+    with an accurate wait rather than a punishment that grows.
+    """
+
+    def __init__(
+        self,
+        limit: int,
+        window_seconds: float,
+        max_keys: int = _MAX_BUCKETS,
+        sweep_interval_seconds: float = _SWEEP_INTERVAL_SECONDS,
+    ) -> None:
+        self._limit = limit
+        self._window_seconds = window_seconds
+        self._max_keys = max_keys
+        self._sweep_interval_seconds = sweep_interval_seconds
+        self._events: dict[str, list[float]] = {}
+        self._last_sweep_at = time.monotonic()
+
+    def _live_events(self, key: str, now: float) -> list[float]:
+        """The key's events still inside the window, pruned in place."""
+        events = self._events.get(key)
+        if events is None:
+            return []
+        cutoff = now - self._window_seconds
+        while events and events[0] <= cutoff:
+            events.pop(0)
+        if not events:
+            del self._events[key]
+        return events
+
+    def _prune(self, now: float) -> None:
+        # Same bound as RateLimiter (B-56): the keys are caller-chosen, so the dict
+        # needs both a sweep and a hard cap. Eviction order is likewise "closest to
+        # leaving the window first" — dropping a key with room left in its quota
+        # changes nothing, dropping a full one hands out free accounts.
+        for key in list(self._events):
+            self._live_events(key, now)
+        self._last_sweep_at = now
+
+        excess = len(self._events) - self._max_keys
+        if excess > 0:
+            by_oldest = sorted(self._events.items(), key=lambda item: item[1][-1])
+            for key, _ in by_oldest[:excess]:
+                del self._events[key]
+
+    def retry_after(self, key: str) -> float:
+        """Seconds until this key may act again — 0 while it is under quota."""
+        now = time.monotonic()
+        events = self._live_events(key, now)
+        if len(events) < self._limit:
+            return 0.0
+        return events[0] + self._window_seconds - now
+
+    def record(self, key: str) -> None:
+        """Counts one *completed* action. Attempts that create nothing (a taken
+        username, a validation error) deliberately don't consume the quota — the
+        limit is on accounts that exist, not on requests."""
+        now = time.monotonic()
+        if now - self._last_sweep_at >= self._sweep_interval_seconds or len(self._events) > self._max_keys:
+            self._prune(now)
+        self._events.setdefault(key, []).append(now)
+
+
+_REGISTRATIONS_PER_IP = 5
+_REGISTRATION_WINDOW_SECONDS = 3600.0
+
+
 class AuthRateLimiters:
     """The three throttles B-33 needs, bundled so they can live on `app.state`
     (like `UserLocks`, see app/tx/locks.py) rather than as module globals.
@@ -124,4 +202,9 @@ class AuthRateLimiters:
     def __init__(self) -> None:
         self.login = RateLimiter(threshold=5, base_delay=2.0, max_delay=300.0)
         self.login_ip = RateLimiter(threshold=20, base_delay=2.0, max_delay=300.0)
-        self.register_ip = RateLimiter(threshold=5, base_delay=5.0, max_delay=600.0)
+        # B-58: a quota, not failure backoff — 5 accounts per IP per hour. The one
+        # over it waits only until the oldest of the five ages out, and a busy NAT
+        # is slowed rather than locked out for progressively longer.
+        self.register_ip = RollingQuota(
+            limit=_REGISTRATIONS_PER_IP, window_seconds=_REGISTRATION_WINDOW_SECONDS
+        )