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.

docs/api/openapi.yaml

@@ -101,7 +101,10 @@ paths:
               schema:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "429": &a5
-          description: Rate limit exceeded.
+          description: Rate limit exceeded. The response carries `Retry-After` (whole seconds, rounded up — wait at least that
+            long before retrying), `X-RateLimit-Limit` (the bucket's cap), `X-RateLimit-Remaining` and
+            `X-RateLimit-Reset` (the reset instant, ISO-8601, not epoch seconds). Back off on `Retry-After` rather than
+            guessing.
           content:
             application/json:
               schema: