Stop storing raw file bytes in media.text_content

`text_content` is an extracted-text cache, but nothing stopped it from
holding a dump of bytes:

- `fileToText` ended its switch with `default: return buffer.toString("utf-8")`
  and defaulted a missing extension to "txt", so any binary was decoded.
- Both extraction callers gated on `media_type == "document"`, and
  `attachmentFileKind` classes everything non-image/audio/video as a
  document: an SVG, a zip, a spreadsheet, a .bin.
- Nothing capped the length, so a 20 MB attachment could leave 20 MB of
  text in the database.
- `storeChatAttachments` copied the text onto a second `media` row, and
  `SELECT m.*` pulled the column into page props and API responses.

On a real install an SVG held 6852 characters of markup and a
`.designcraft` file held 142 KB of raw bytes.

Changes:

- `file-to-text.ts` lists the text extensions explicitly and returns
  undefined for anything else; no extension is no text.
- `can-extract-text.ts` is the one allowlist (reusing
  `BotTextFileExtensions`), used by both upload routes and by
  `uploadAndRecordMedia`; `extension-of.ts` is shared with the storage
  catalog instead of duplicated.
- `BotAttachmentTextStoreMaxChars` (200k) caps the column via
  `cap-text-content.ts`.
- `store-chat-attachments.ts` no longer copies the text; ingestion
  re-reads the bytes once.
- `media-columns.ts` selects every media column but `text_content`, which
  now has to be asked for (`grab-messages-media`'s reply path and
  ingestion do); the media CRUD routes sanitize rows they write.
- `ingest-bot-file.ts` decides on size alone, since a cached slice made
  an oversized file look already read.
- `bun run media:prune-text` cleans rows that were already stored
  (rerunnable, with a VACUUM hint).

Verified on the dev database: both bad rows cleared (146 KB), rerun
reports nothing to do.

src/functions/backend/db/media/sanitize-media-row-text-content.ts

@@ -0,0 +1,49 @@
+import type { BUN_SQLITE_COBBLE_MEDIA } from "@/db/types/db";
+import capTextContent from "@/src/utils/cap-text-content";
+import decideTextContent from "@/src/utils/decide-text-content";
+
+type Params = {
+    /**
+     * The row as the request sent it.
+     */
+    row: Partial<BUN_SQLITE_COBBLE_MEDIA>;
+    /**
+     * The row already in the database, for an update: its name and type
+     * decide whether the new text may be kept when the update doesn't
+     * repeat them.
+     */
+    existing?: Partial<BUN_SQLITE_COBBLE_MEDIA> | null;
+};
+
+/**
+ * A `media` row whose `text_content` is what the column may hold.
+ *
+ * The upload path decides this itself, but the CRUD API writes rows
+ * straight through, so a typed-in `text_content` could store raw bytes or
+ * megabytes of text. An update that carries no text is left alone.
+ */
+export default function sanitizeMediaRowTextContent({
+    row,
+    existing,
+}: Params): Partial<BUN_SQLITE_COBBLE_MEDIA> {
+    const text = row.text_content;
+
+    if (text === undefined) return row;
+
+    const name = row.title || existing?.title || null;
+    const mime_type = row.mime_type || existing?.mime_type || null;
+
+    /**
+     * With no name and no type there is nothing to judge it by, so it is
+     * only cut: dropping text that a rename would have kept would be
+     * worse than a slightly generous cache.
+     */
+    const text_content =
+        name || mime_type
+            ? decideTextContent({ name, mime_type, text })
+            : text
+              ? capTextContent({ text })
+              : null;
+
+    return { ...row, text_content: text_content ?? undefined };
+}