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>

test/routes/file-routes-range.test.ts

@@ -191,7 +191,9 @@ describe('file-raw range requests', () => {
   });
 
   it('still refuses files past the raw size cap before looking at Range', async () => {
-    mockedStat.mockResolvedValue({ size: 100 * 1024 * 1024, isFile: () => true } as never);
+    // 3GB, past the 2GB CODEMAN_MAX_DOWNLOAD_BYTES default. The cap is checked
+    // before the range, so a small slice of an oversized file is refused too.
+    mockedStat.mockResolvedValue({ size: 3 * 1024 * 1024 * 1024, isFile: () => true } as never);
 
     const res = await harness.app.inject({
       method: 'GET',
@@ -199,6 +201,18 @@ describe('file-raw range requests', () => {
       headers: { range: 'bytes=0-99' },
     });
 
-    expect(res.statusCode).toBe(400);
+    expect(res.statusCode).toBe(413);
+  });
+
+  it('serves a 100MB file that the historical 50MB cap would have refused', async () => {
+    mockedStat.mockResolvedValue({ size: 100 * 1024 * 1024, isFile: () => true } as never);
+
+    const res = await harness.app.inject({
+      method: 'GET',
+      url: rawUrl('big.mp4'),
+      headers: { range: 'bytes=0-99' },
+    });
+
+    expect(res.statusCode).toBe(206);
   });
 });

test/routes/file-routes.test.ts

@@ -802,14 +802,27 @@ describe('file-routes', () => {
       expect(res.statusCode).toBe(404);
     });
 
-    it('rejects overly large raw files', async () => {
-      mockedStat.mockResolvedValue({ size: 100 * 1024 * 1024 } as never); // 100MB
+    it('serves a file past the historical 50MB cap', async () => {
+      // The body is streamed and Range-aware, so size costs a read stream, not
+      // RSS. The old 50MB refusal only blocked legitimate artifact downloads.
+      mockedStat.mockResolvedValue({ size: 100 * 1024 * 1024, isFile: () => true } as never); // 100MB
 
       const res = await harness.app.inject({
         method: 'GET',
         url: `/api/sessions/${harness.ctx._sessionId}/file-raw?path=huge.bin`,
       });
-      expect(res.statusCode).toBe(400);
+      expect(res.statusCode).toBe(200);
+    });
+
+    it('still refuses a file past the configured download cap', async () => {
+      mockedStat.mockResolvedValue({ size: 3 * 1024 * 1024 * 1024, isFile: () => true } as never); // 3GB > 2GB default
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: `/api/sessions/${harness.ctx._sessionId}/file-raw?path=enormous.bin`,
+      });
+      expect(res.statusCode).toBe(413);
+      expect(JSON.parse(res.body).error).toContain('CODEMAN_MAX_DOWNLOAD_BYTES');
     });
   });
 
@@ -863,8 +876,9 @@ describe('file-routes', () => {
     });
 
     it('downloads files scoped to the session working directory', async () => {
-      const content = Buffer.from('download content');
-      mockedReadFile.mockResolvedValue(content as never);
+      // The body is streamed (shared sendFileBody path), so the bytes come from
+      // the createReadStream mock rather than from readFile.
+      const content = Buffer.from('fake file bytes');
       mockedStat.mockResolvedValue({ size: content.length, isFile: () => true } as never);
 
       const res = await harness.app.inject({
@@ -874,7 +888,19 @@ describe('file-routes', () => {
 
       expect(res.statusCode).toBe(200);
       expect(res.headers['content-disposition']).toContain('filename="report.txt"');
-      expect(res.body).toBe('download content');
+      expect(res.headers['accept-ranges']).toBe('bytes');
+      expect(res.body).toBe('fake file bytes');
+    });
+
+    it('refuses a download past the configured cap', async () => {
+      mockedStat.mockResolvedValue({ size: 3 * 1024 * 1024 * 1024, isFile: () => true } as never); // 3GB > 2GB default
+
+      const res = await harness.app.inject({
+        method: 'GET',
+        url: `/api/download?sessionId=${harness.ctx._sessionId}&path=enormous.bin`,
+      });
+
+      expect(res.statusCode).toBe(413);
     });
 
     it('rejects absolute paths outside the session working directory', async () => {