fix: bound browser upload chunk reads

[BrandonRobare]

Closes #585

Problem

Browser-upload, flight-log, and SRT handlers allocated entire multipart payloads before enforcing configured size limits.

Changes

• add one shared limit + 1 upload reader
• use it in browser chunks, flight logs, and SRT uploads
• preserve existing HTTP 413 messages and browser state/path/offset ordering
• add helper and actual multipart browser-endpoint regressions

Verification

• focused: 31 passed
• full suite: 1,054 passed, 5 skipped
• Ruff and diff checks passed
• sabotage of browser endpoint back to await chunk.read() failed the endpoint regression as required
• repaired exact SHA independently accepted: 3ae55d027676483d10f13695dd658468c608a81e