feat(files): raise the download cap to 2GB and stream /api/download

The 50MB cap on file-raw, the attachment /raw route and /api/download was
memory protection for a `readFile()` that no longer exists: file-raw and
/raw were rewritten to stream through `sendFileBody()` and answer Range
requests, so size costs a read stream rather than RSS (measured: a 600MB
download moved peak RSS by ~37MB). All the cap still did was refuse
legitimate downloads of build artifacts, videos and archives.

It is now MAX_FILE_DOWNLOAD_BYTES in config/buffer-limits.ts, default 2GB,
env CODEMAN_MAX_DOWNLOAD_BYTES, 0 = unlimited. `parseByteLimitEnv()` is
separate from the `parseInt(...) || default` idiom used elsewhere in that
file precisely because that idiom reads 0 as falsy and would silently
restore the default for the one value that means "no limit".

/api/download was the last route that really did buffer the whole file. It
now shares sendFileBody() with the other two, so it streams, advertises
Accept-Ranges, and is resumable. Its Content-Disposition also goes through
buildContentDisposition() rather than raw interpolation.

Refusals move from 400 to 413 across all three, which is the correct status
for the case; with the cap at 2GB it is a path almost nothing reaches now.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/wiki/Working-With-Files.md

@@ -19,7 +19,9 @@ It renders what it can:
 | PDF and Office documents | Converted for preview when a converter is available.                       |
 | Anything else            | Download.                                                                  |
 
-Caps: 10 MB for text preview, 50 MB for raw and download. Sensitive paths (`.env`, anything
+Caps: 10 MB for text preview, 2 GB for raw and download (set `CODEMAN_MAX_DOWNLOAD_BYTES`
+to change it, `0` for no limit — these bodies are streamed, so a large file costs a read
+stream rather than server memory). Sensitive paths (`.env`, anything
 matching credentials, `~/.ssh`, AWS credentials) are blocked from download, and SVG and HTML
 are served as downloads rather than rendered, so they cannot execute in the page.