security: rate-limit IP-keyed, fail-closed on empty key (#37)

Rate-limiter now accepts string | string[] so callers can key on
multiple buckets simultaneously. If any bucket is exhausted the
request is denied, which lets login/TOTP/reset-password throttle on
BOTH user identifier and source IP without either becoming a bypass.

Fail-closed: empty/whitespace-only keys now deny by default instead
of silently allowing unbounded attempts (was CWE-307 gap).

Degraded-fallback divisor reduced from /10 to /2 — the old aggressive
clamp forced-logged-out legitimate users during brief Redis outages;
/2 still meaningfully slows distributed brute-force.

Callers updated:
- auth.ts (login): both email: and ip: buckets
- auth router requestPasswordReset: email + IP
- auth router resetPassword: IP before lookup, email-reset after
- invite router getInvite/acceptInvite: IP
- user-self-service verifyTotp: userId + IP

TRPCContext now carries clientIp; web tRPC route extracts it from
X-Forwarded-For / X-Real-IP.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/api/src/router/assistant-tools/helpers.ts

@@ -1677,6 +1677,7 @@ export function createScopedCallerContext(ctx: ToolContext): TRPCContext {
     db: ctx.db,
     dbUser: ctx.dbUser,
     roleDefaults: ctx.roleDefaults ?? null,
+    clientIp: null,
   };
 }