docs: README and server placeholder

README covers how the pieces fit: the storage interface, the inverted
frequency join, the gate's three refinements, and the checks that guard
each one.

server/ holds a README only. Steps 1-3 give a fully working offline app with
no server at all, and that is the version that gets used first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

server/README.md

@@ -0,0 +1,25 @@
+# server/ — not in this pass
+
+PORT.md steps 4–6 land here: the sync API, the tutor SSE endpoint, and the
+Docker Compose deployment beside the Pi's existing Postgres and Caddy.
+
+Nothing here yet, on purpose. Steps 1–3 give a fully working offline app with
+no server at all, and that is the version that gets used first.
+
+What is already shaped for it:
+
+- every syncable table carries `updated_at`, and seeded rows are pinned to 0
+  (see `app/src/db/writes.ts`) — the artifact's clobbering bug cannot be
+  expressed;
+- `change_seq` is deliberately absent: it is server-assigned and arrives with
+  the sync layer;
+- the dictionary is fetched by URL from `app/src/domain/dictionary.ts`, so
+  per-band deltas can come from the Pi instead of the bundle by changing a base
+  URL;
+- the tutor goes through one `Sample` function
+  (`app/src/domain/stub-tutor.ts`). The real endpoint implements the same
+  contract — `onText` receives cumulative text, and an aborted turn stops
+  billing — so only that file changes.
+
+When the SSE endpoint lands, Caddy needs `flush_interval -1` on the proxy or
+the stream buffers and the tutor appears to hang.