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/plan-text-content-prune.ts

@@ -0,0 +1,55 @@
+import type { BUN_SQLITE_COBBLE_MEDIA } from "@/db/types/db";
+import decideTextContent from "@/src/utils/decide-text-content";
+
+export type TextContentChange = {
+    media_id: number;
+    name: string;
+    before_chars: number;
+    after_chars: number;
+    /**
+     * What the row's `text_content` should become: `null` clears it.
+     */
+    text_content: string | null;
+};
+
+type Params = {
+    /**
+     * The `media` rows that have any `text_content`.
+     */
+    rows: Partial<BUN_SQLITE_COBBLE_MEDIA>[];
+};
+
+/**
+ * What `pruneMediaTextContent` would change, worked out without touching
+ * the database: one entry per row whose text belongs nowhere or is over
+ * the store ceiling, and nothing at all for a row that is already right.
+ */
+export default function planTextContentPrune({ rows }: Params): TextContentChange[] {
+    const changes: TextContentChange[] = [];
+
+    for (const row of rows) {
+        if (!row.id) continue;
+
+        const before = row.text_content || "";
+
+        if (!before.trim()) continue;
+
+        const text_content = decideTextContent({
+            name: row.title,
+            mime_type: row.mime_type,
+            text: before,
+        });
+
+        if (text_content === before) continue;
+
+        changes.push({
+            media_id: Number(row.id),
+            name: row.title || "no name",
+            before_chars: before.length,
+            after_chars: text_content?.length || 0,
+            text_content,
+        });
+    }
+
+    return changes;
+}