fix(api): put Retry-After and the rate-limit triple on every 429

A refused client had nothing to back off against. `rateLimitHeaders()`
emitted `X-RateLimit-Remaining` and an ISO `X-RateLimit-Reset`, no
`Retry-After` and no cap, and only about 70 of the 199 route files that
consult a limiter attached even those two. The rest answered with a bare
prose envelope, including every endpoint an offline replay queue hits:
the four batch ingests, `/api/sync/changes` and `/api/auth/refresh`. A
first-run Apple Health import trips the 60-per-minute batch ceiling
reliably, and the only strategy left to the client was a blind backoff.

Threading headers through 129 handlers is 129 edits the next new route
forgets, so the verdict is carried instead: `checkRateLimit` records it
in a request-scoped cell and `apiHandler` dresses any 429 that leaves
without headers. A route that already built its own is left alone —
`Retry-After` is the presence test, and it comes from the same builder.

`Retry-After` is whole seconds rounding UP (RFC 9110 §10.2.3), floored
at zero. `X-RateLimit-Limit` is new and needs the cap, so the limiter
result carries it. `X-RateLimit-Reset` keeps its ISO form: it is the
wrong convention for the name, but it is on the wire today and changing
it is a decision of its own.

The cell lives in its own leaf module because 177 test files replace
`@/lib/rate-limit` wholesale with a mock factory, and `apiHandler` must
not depend on anything those factories stand in for.

src/__tests__/api/auth/check-user/route.test.ts

@@ -27,6 +27,7 @@ vi.mock("@/lib/logging/transports", () => ({ emitIfSampled: vi.fn() }));
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 29,
     resetAt: Date.now() + 15 * 60 * 1000,
   }),
@@ -34,6 +35,7 @@ vi.mock("@/lib/rate-limit", () => ({
   // wrapper. Default to a clean per-IP result.
   checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 29,
     resetAt: Date.now() + 15 * 60 * 1000,
     ip: "203.0.113.1",
@@ -84,11 +86,13 @@ beforeEach(() => {
   vi.resetAllMocks();
   vi.mocked(checkRateLimit).mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 29,
     resetAt: Date.now() + 15 * 60 * 1000,
   });
   vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 29,
     resetAt: Date.now() + 15 * 60 * 1000,
     ip: "203.0.113.1",
@@ -175,6 +179,7 @@ describe("POST /api/auth/check-user", () => {
   it("returns 429 when the per-IP rate-limit is exhausted", async () => {
     vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 15 * 60 * 1000,
       ip: "203.0.113.1",