Document the translation dispatcher and capability handshake

docs/migrate.md

@@ -55,6 +55,9 @@ class Data(msgspec.Struct):
     #: API key gating the translator service WebSocket (/_translate/{key});
     #: generated lazily at startup (see the lifespan in app.py).
     translate_key: str = ""
+    #: Wanted target languages for the translator service (presence-keys);
+    #: jobs are offered only in these ∩ a connection's capabilities.
+    translate_langs: dict[str, True] = {}
     #: All original-language text, content-addressed: blake3(normalized)
     #: digest[:9] -> Markdown chunk. Shared by every article. Keys are
     #: bytes; kanta/msgspec base64-encode them at the JSON level.
@@ -94,11 +97,13 @@ alternate links never enumerate chunks. It is written by whoever writes
 translation data, in the same transaction:
 
 - **Translator service:** the WebSocket API at `/_translate/{key}` (see
-  docs/localization.md) pushes pending fragments (titles + translatable
-  chunks lacking an entry, deduped by hash) and receives result batches;
-  storing a batch writes `trans[h][lang]` entries, sets
-  `node.langs[lang] = True` on every article that gained one and
-  invalidates the page cache — all in one transaction.
+  docs/localization.md) offers pending fragments (titles + translatable
+  chunks lacking an entry for the language) as single-item jobs — one at
+  a time per connection, in `Data.translate_langs` ∩ the connection's
+  announced capabilities — and receives the matching result; storing it
+  writes the `trans[h][lang]` entry, sets `node.langs[lang] = True` on
+  every article that gained one and invalidates the page cache — all in
+  one transaction.
 - **Translated-view save:** appending the first patch for `f"{path}:{lang}"`
   sets `node.langs[lang] = True` (patches alone make the version exist).
 - **Removals:** deleting a patch or GC'ing translations re-derives the key: