feat(insights): day navigator on the intraday pulse chart; 90-day dense retention with hourly fallback

Adds prev/next-day paging to the intraday pulse "shape of the day" chart
(TileHeader right slot, next disabled at today) so a user can step back
through prior days on the existing date-scoped route.

Raises DENSE_INTRADAY_RETENTION_DAYS from 14 to 90 — a self-hoster's own
storage is the trade-off for three months of full 10-minute-resolution
history instead of two weeks. Days outside the window keep folding to
hourly-mean stats: rows as before; loadIntradayPulse now falls back to
that hourly tier when a day's raw samples are gone, so the navigator
shows a coarser shape instead of an empty chart. The tension-window
read is suppressed on hourly days — it needs per-sample resolution to
stay honest. A new resolution field on the DTO ("tenMin" | "hourly")
drives a small note in the chart.

src/app/api/insights/pulse/intraday/route.ts

@@ -10,7 +10,11 @@
  * Cookie OR Bearer auth via `requireAuth()`; `userId` is narrowed from the
  * resolved session — never a body field. Gated on the `insights` module. The
  * `date` defaults to the user's local today; anything but a `YYYY-MM-DD`
- * literal is a 422 via `returnAllZodIssues`.
+ * literal is a 422 via `returnAllZodIssues`. The frontend day navigator
+ * pages this same route backward through prior days — a day outside
+ * `DENSE_INTRADAY_RETENTION_DAYS` (`dense-intraday-retention.ts`) reads back
+ * at the coarser hourly grain instead of an empty series; the response's
+ * `resolution` field tells the caller which.
  */
 import { z } from "zod";
 
@@ -54,6 +58,7 @@ export const GET = apiHandler(async (req: Request) => {
       buckets: result.series.length,
       baseline_source: result.baselineSource,
       tension: result.tension !== null,
+      resolution: result.resolution,
     },
   });