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/__tests__/user-self-service-mfa.test.ts

@@ -90,15 +90,16 @@ function makeSelfServiceCtx(dbOverrides: Record<string, unknown> = {}) {
   };
 }
 
-function makePublicCtx(dbOverrides: Record<string, unknown> = {}) {
+function makePublicCtx(overrides: Record<string, unknown> = {}) {
   return {
     db: {
       user: {
         findUnique: vi.fn(),
         update: vi.fn().mockResolvedValue({}),
-        ...((dbOverrides.user as object | undefined) ?? {}),
+        ...((overrides.user as object | undefined) ?? {}),
       },
     },
+    clientIp: (overrides.clientIp as string | null | undefined) ?? null,
   };
 }
 
@@ -277,14 +278,27 @@ describe("verifyTotp", () => {
     expect(ctx.db.user.findUnique).not.toHaveBeenCalled();
   });
 
-  it("calls the rate limiter with the userId as key", async () => {
+  it("calls the rate limiter with both userId and client IP as keys", async () => {
+    totpValidateMock.mockReturnValue(0);
+    const ctx = makePublicCtx({
+      user: { findUnique: vi.fn().mockResolvedValue(mfaUser) },
+      clientIp: "198.51.100.7",
+    });
+    await verifyTotp(ctx as Parameters<typeof verifyTotp>[0], {
+      userId: "user_1",
+      token: "123456",
+    });
+    expect(totpRateLimiterMock).toHaveBeenCalledWith(["user:user_1", "ip:198.51.100.7"]);
+  });
+
+  it("falls back to userId-only keying when no client IP is available", async () => {
     totpValidateMock.mockReturnValue(0);
     const ctx = makePublicCtx({ user: { findUnique: vi.fn().mockResolvedValue(mfaUser) } });
     await verifyTotp(ctx as Parameters<typeof verifyTotp>[0], {
       userId: "user_1",
       token: "123456",
     });
-    expect(totpRateLimiterMock).toHaveBeenCalledWith("user_1");
+    expect(totpRateLimiterMock).toHaveBeenCalledWith(["user:user_1"]);
   });
 });