feat(gateway): capture and persist streamed prompts

[kev1n77]

Summary

• add bounded, nonblocking request and response stream capture to the local AI Gateway
• extend the capture contract with Gateway source and truncated body states, plus queue/degradation metrics
• process request events through Capture Policy, the adapter registry, SQLCipher persistence, and the Desktop API
• cover exact forwarding, truncation, queue pressure, cancellation, empty bodies, credential scrubbing, and encrypted persistence

Verification

• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --manifest-path apps/desktop/src-tauri/Cargo.toml -- --check
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• npm run desktop:check
• npm run desktop:build
• python -m unittest discover -s sidecars/mitmproxy/tests -v

Scope note

Response and upstream-failure events are surfaced as structured observations. Persisting response data is intentionally deferred until the storage response schema is introduced.