Add knowledgebases

Knowledgebases give bots RAG over documents, scoped like extensions: to
particular bots, to a workspace, or to every bot, with their own button
beside Tools in the composer. Documents are chunked and embedded by the
file-ingest worker with the install's embedding model, which is now an
install-wide setting rather than a per-bot one.

- Word search runs on an FTS5 index ranked by BM25, with a scored LIKE
  fallback for scripts written without spaces.
- Changing the embedding model re-indexes documents automatically.
- Ingest jobs wait without spending attempts while every embedding model
  rests, fail at once on errors retrying can't fix, and keep passages
  searchable by words when the model refuses.
- Settings > Knowledgebases shows each knowledgebase's and document's
  index state: spinner, warning, grey check (words only) or green check.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/pages/admin/settings/knowledgebases/(functions)/read-file-data-url.ts

@@ -0,0 +1,16 @@
+type Params = {
+    file: File;
+};
+
+/**
+ * A picked file as a data URL, for an upload. Empty when the browser can't
+ * read it.
+ */
+export default async function readFileDataUrl({ file }: Params) {
+    return await new Promise<string>((resolve, reject) => {
+        const reader = new FileReader();
+        reader.onload = () => resolve(String(reader.result || ""));
+        reader.onerror = () => reject(reader.error);
+        reader.readAsDataURL(file);
+    }).catch(() => "");
+}