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/utils/can-extract-text.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, test } from "bun:test";
+import canExtractText from "./can-extract-text";
+
+describe("canExtractText", () => {
+    test("accepts the types the extractor really unpacks", () => {
+        for (const name of ["a.pdf", "b.docx", "c.txt", "d.md", "e.csv", "f.tsv", "g.json", "h.html", "i.xml", "j.yml", "k.yaml", "l.log", "m.markdown"]) {
+            expect(canExtractText({ name })).toBe(true);
+        }
+    });
+
+    test("refuses a binary that only has bytes, not text", () => {
+        for (const name of ["icon.svg", "bundle.zip", "sheet.xlsx", "slides.pptx", "app.bin", "noext", "archive.tar.gz"]) {
+            expect(canExtractText({ name })).toBe(false);
+        }
+    });
+
+    test("reads the type when the name has no extension", () => {
+        expect(canExtractText({ name: "noext", mime_type: "application/pdf" })).toBe(true);
+        expect(canExtractText({ name: "noext", mime_type: "image/svg+xml" })).toBe(false);
+    });
+
+    test("an octet-stream says nothing, so the name decides", () => {
+        expect(canExtractText({ name: "notes.md", mime_type: "application/octet-stream" })).toBe(true);
+        expect(canExtractText({ name: "logo.svg", mime_type: "application/octet-stream" })).toBe(false);
+    });
+});