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/auth/me/diabetes/__tests__/route.test.ts

@@ -32,6 +32,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -192,6 +193,7 @@ describe("PATCH /api/auth/me/diabetes", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });

src/app/api/auth/me/disable-coach/__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: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -247,6 +248,7 @@ describe("PATCH /api/auth/me/disable-coach", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });

src/app/api/auth/me/glucose-unit/__tests__/route.test.ts

@@ -36,6 +36,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -197,6 +198,7 @@ describe("PATCH /api/auth/me/glucose-unit", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });

src/app/api/auth/me/health-score-config/__tests__/route.test.ts

@@ -35,6 +35,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -397,6 +398,7 @@ describe("PATCH — envelope and transport", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });

src/app/api/auth/me/modules/__tests__/route.test.ts

@@ -35,6 +35,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -300,6 +301,7 @@ describe("PATCH /api/auth/me/modules", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });

src/app/api/auth/me/notification-prefs/__tests__/route.test.ts

@@ -26,6 +26,7 @@ vi.mock("@/lib/db-compat", () => ({
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn().mockResolvedValue({
     allowed: true,
+    limit: 60,
     remaining: 59,
     resetAt: Date.now() + 60_000,
   }),
@@ -587,6 +588,7 @@ describe("PATCH /api/auth/me/notification-prefs", () => {
     vi.mocked(getSession).mockResolvedValue(SESSION_OK as never);
     vi.mocked(checkRateLimit).mockResolvedValueOnce({
       allowed: false,
+      limit: 60,
       remaining: 0,
       resetAt: Date.now() + 30_000,
     });