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/app/api/user/avatar/__tests__/route.test.ts

@@ -24,6 +24,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 9,
     resetAt: Date.now() + 3_600_000,
   }),
@@ -89,6 +90,7 @@ beforeEach(() => {
   // Default rate-limit pass-through; per-test overrides drop in below.
   vi.mocked(checkRateLimit).mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 9,
     resetAt: Date.now() + 3_600_000,
   });
@@ -210,6 +212,7 @@ describe("POST /api/user/avatar", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });