downloads: track every door and surface them on /analytics

The analytics page could not answer "how many downloads, of which
product". Three of the four download doors recorded nothing: the tRPC
click proxy only left the generic request row, whose path is the raw URL
with the asset buried in `?batch=1&input={"asset_name":...}`, and the S3
file route that serves Sparkle updates and the Nook app logged nothing
at all. The one door that did log (the Nook redirect) kept a private copy
of the logVisit/enrich/flush sequence, under its own path.

All doors now go through trackDownload, which writes one row per event
under /api/downloads/<asset> and flushes it inside the request — a click
is low-volume and serverless is not guaranteed to run the 30s batch
timer after the response. The file route maps its prefix allowlist to
asset keys, so a version bump stays one asset rather than one per
release. getAnalyticsSummary groups those rows into a downloads block and
the freno.me panel renders it under the overview cards.

History is recovered to the degree it exists: the Nook door's rows and
the tRPC clicks still carry enough to derive from (asset name in the URL).
scripts/backfill-downloads.ts inserts those derived rows with a
`backfill-<source id>` primary key and INSERT OR IGNORE, leaving the
originals untouched, so re-running is a no-op. Run against the live table
it added 66 rows: gaze 42, thenook 19, inputhalo 4, lineage 1.

Tests: new backfill suite (derivation, visitor detail carried over,
idempotency, empty history) and summariseDownloads cases; the Nook route
test now pins the canonical path, and the NOOK-337 allowlist audit reads
the map keys instead of quoted strings. Red-checked by stubbing the
downloads block back to zeros — the summary case fails, then passes.
Full suite matches master.

Co-Authored-By: nw-flash <noreply@nw-flash.local>
MISC-48

scripts/backfill-downloads.ts

@@ -0,0 +1,21 @@
+import { ConnectionFactory } from "~/server/db-connections";
+import { backfillDownloads } from "~/server/backfill-downloads";
+
+// One-off: derive canonical /api/downloads/<asset> rows from download history
+// recorded before `trackDownload` existed. Idempotent — re-running adds
+// nothing, because derived rows carry a `backfill-<source id>` primary key.
+//
+//   bun --env-file=.env scripts/backfill-downloads.ts
+
+const result = await backfillDownloads(ConnectionFactory());
+
+if (result.length === 0) {
+  console.log("Nothing to backfill: no legacy download rows left to derive.");
+} else {
+  for (const { asset, added } of result) {
+    console.log(`${asset}: +${added}`);
+  }
+  console.log(
+    `Total: +${result.reduce((sum, row) => sum + row.added, 0)} derived download rows.`
+  );
+}