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/harness/media/read-media.ts

@@ -1,3 +1,4 @@
+import { BotTextFileExtensions } from "@/src/dict/bot-file-dict";
 import fileToText from "@/src/functions/backend/db/users/media/file-to-text";
 import AppError from "@/src/utils/app-error";
 import transcribeAudio from "../audio/transcribe-audio";
@@ -24,7 +25,11 @@ type Params = {
     workspace_id?: number | string | null;
 };
 
-const DocumentExtensions = ["pdf", "docx", "txt", "md", "csv", "json", "html", "xml", "yaml", "yml"];
+/**
+ * One list, the same one the local extractor reads: keeping a second copy
+ * here is how a type got read by one and silently missed by the other.
+ */
+const DocumentExtensions: readonly string[] = BotTextFileExtensions;
 
 const TypeExtensions: Record<string, string> = {
     "application/pdf": "pdf",