feat: serve the BYOS device API

The panel now pairs, fetches its image and files its logs against this
application rather than against the TRMNL cloud.

Four endpoints: /api/setup issues a token on first contact,
/api/display hands back an image and a wake interval, /api/log stores
firmware diagnostics, and /api/device/image/<hash> serves the bytes.

The wake interval is where freshness and battery are traded off. In BYOS
nothing can be pushed: the device sleeps, wakes, asks and sleeps again.
So the interval is short while the shop trades and long overnight, and
it is shortened further whenever a change of state falls inside it —
the door opening in twenty minutes means waking in twenty-one,
whatever the base interval says.

The image filename is the hash of its own bytes. The firmware skips the
redraw when the name is unchanged, which is the whole battery strategy,
and the URL is immutable, unguessable and safe to cache forever. Two
integration tests pin this: unchanged data must yield the same filename
and store one row, changed hours must yield a different one.

MAC addresses are normalised before use. They are a primary key here,
and firmwares are inconsistent about case and separators; without this a
panel could register twice by capitalising itself differently. Header
names are read in both the hyphen and underscore spellings for the same
reason — the TRMNL docs and the Seeed sources disagree, and being
liberal costs nothing while being wrong costs a blank shop window.

Pairing is deliberately made to survive a rendering failure. The token
is issued once and only its digest is kept, so a device stranded by a
failed response would be registered yet hold no credential, and unable
to register again. The welcome image is worth far less than that. This
was found by running the flow, not by reading it.

satori, yoga and harfbuzz are marked external: bundling rewrites the
relative path satori uses to load its WebAssembly, and the renderer dies
on a missing hb.wasm.

The integration tests run against a real Postgres, in CI too. Mocking
Prisma here would only prove the mock works.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012cSY9pVhZmJUKNN7wf1Myd

prisma/migrations/20260920155146_screen_images/migration.sql

@@ -0,0 +1,12 @@
+-- CreateTable
+CREATE TABLE "screen_images" (
+    "hash" TEXT NOT NULL,
+    "format" TEXT NOT NULL,
+    "bytes" BYTEA NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "screen_images_pkey" PRIMARY KEY ("hash")
+);
+
+-- CreateIndex
+CREATE INDEX "screen_images_createdAt_idx" ON "screen_images"("createdAt");

prisma/schema.prisma

@@ -220,6 +220,23 @@ model DeviceLog {
   @@map("device_logs")
 }
 
+/// A rendered panel image, addressed by the hash of its own bytes.
+///
+/// Stored rather than regenerated on demand because the device fetches the
+/// image in a second request, moments after being told its name: a restart or
+/// a data change in between would otherwise hand it a 404. Old rows are pruned
+/// by the daily job.
+model ScreenImage {
+  /// SHA-256 of `bytes`, truncated; also the filename given to the firmware.
+  hash      String   @id
+  format    String
+  bytes     Bytes
+  createdAt DateTime @default(now())
+
+  @@index([createdAt])
+  @@map("screen_images")
+}
+
 /// Last outcome of each background sync, so the UI can show "last successful
 /// sync" instead of failing silently.
 model SyncState {