feat: configure HTTP timeouts

[trchopan]

Summary

• Add per-HTTP-service upstream_timeout_secs configuration, defaulting to 60 seconds and bounded to 1-300 seconds.
• Add per-attachment http_timeout_secs configuration and positional --http-timeout-secs support.
• Enforce host timeouts across the complete forwarding lifecycle with one absolute deadline.
• Handle attachment response timeouts as 504 Gateway Timeout and invalidate incomplete shared tunnel leases.
• Add boundary, CLI, TOML, post-header, recovery, release, performance, and soak coverage.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• 68 unit tests passed
• 19 integration tests passed
• Release smoke tests passed against release binaries
• Python load-generator tests passed
• Docker Compose configuration and performance scripts validated
• scripts/verify-release.sh --skip-artifacts passed

Performance and soak

• 15-minute supported-load stress baseline passed with zero failures.
• HTTP concurrency levels: 1, 10, 50, 100.
• TCP concurrency levels: 1, 10, 50.
• 1-hour ARM64 soak passed with 58,680,003 successful operations.
• Soak recorded zero timeout failures and successful host, HTTP attachment, and TCP attachment recovery probes.
• Resource samples showed stable task/file-descriptor counts and bounded memory.
• TCP concurrency 100 was validated separately as an explicit capacity-limit scenario; expected TCP connection limit reached rejections were observed, so it is not part of the standard baseline.

Artifacts from the validation runs are retained locally under artifacts/.