fix: normalize transcription endpoints and asset uploads

[GodsQuantum]

Summary

• accept OpenAI-compatible base URLs ending in /v1 for both model discovery and transcription
• keep full /audio/transcriptions URLs backward-compatible
• stream multipart asset uploads beyond Axum's default body limit while preserving AUTOSUBS_MAX_UPLOAD_BYTES
• release as 3.1.2
• update locked rustls and chacha20 versions so the current RustSec audit is clean

Verification

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features — 77 passed
• cargo audit --file Cargo.lock — clean
• npm audit --audit-level=low — clean
• npm test — 26 passed
• npm run check -- --fail-on-warnings --output machine-verbose — 0 errors / 0 warnings
• npm run build

Cloud9 production compose mount changes are intentionally deployment-specific and are not hardcoded into the public example.