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/users/media/file-to-text.ts

@@ -36,9 +36,15 @@ export default async function fileToText({
         buffer = file_path
             ? Buffer.from(await Bun.file(file_path).arrayBuffer())
             : file_stream;
-        ext = (file_type || file_path?.split(".").pop() || "txt")
-            .toString()
-            .toLowerCase();
+        const known_ext = file_type || file_path?.split(".").pop();
+
+        /**
+         * No extension and no type: nothing says this is text, so nothing
+         * gets decoded. Guessing "txt" is how a binary became text.
+         */
+        if (!known_ext) return undefined;
+
+        ext = known_ext.toString().toLowerCase();
     }
 
     if (!buffer) return undefined;
@@ -76,9 +82,32 @@ export default async function fileToText({
         //     }).join("\n\n");
         // }
 
-        case "md":
+        /**
+         * The text types read as they are. Listed one by one, never a
+         * catch-all: decoding a zip, an SVG or a spreadsheet as UTF-8
+         * fills the media row with raw bytes instead of text.
+         */
         case "txt":
-        default:
+        case "text":
+        case "md":
+        case "markdown":
+        case "csv":
+        case "tsv":
+        case "json":
+        case "html":
+        case "htm":
+        case "xml":
+        case "yml":
+        case "yaml":
+        case "log":
             return buffer.toString("utf-8");
+
+        default:
+            /**
+             * Nothing readable locally: the caller decides whether to read
+             * it another way (an image or audio through the harness) or to
+             * leave its text unset.
+             */
+            return undefined;
     }
 }