Respect Discord attachment limits

[itsreverence]

What changed

• enforce Discord's default 10 MiB limit for each attachment
• retain a separate 24 MiB aggregate media budget so multipart overhead stays under the 25 MiB request limit
• select the 3.7 MB video variant for the reported post instead of the rejected 15.3 MB variant
• update aggregate-budget regression coverage
• bump the release version to 0.6.31 and rebuild the userscript

Root cause

v0.6.30 treated Discord's 25 MiB message request limit as though it were the per-file attachment limit. Discord currently applies a default 10 MiB limit to each webhook attachment, so the selected 15.3 MB MP4 was rejected with Request entity too large.

User impact

The reported video now uploads through the clean native Discord player using the highest-quality MP4 variant that fits the default webhook attachment limit. The CDN link remains hidden when an eligible variant is available.

Validation

• npm test — 98 tests passed
• npm run check
• git diff --check
• exact regression: 15,319,401-byte variant skipped, 3,744,793-byte variant selected
• per-attachment and aggregate request budgets covered

Trust and release impact

• no webhook URL, storage, grant, or connected-host changes
• reduces maximum media upload size; no new network behavior
• generated userscript rebuilt from source
• merging publishes v0.6.31 through the existing release workflow