chore: modernize Rust workspace and CI baseline

[Freyskeyd]

Summary

Modernize the Rust workspace and establish a reliable CI baseline for CHE-6.

• Move the workspace to Rust 2024 with resolver 3 and MSRV 1.88.
• Centralize workspace package and dependency metadata, and narrow broad dependency features.
• Upgrade SQLx to 0.8, Actix to 0.13/4, Inventory to 0.2, and replace TUI with Ratatui.
• Update SQLx executor usage and proc-macro-generated registrations for the newer dependency APIs while preserving the declared MSRV.
• Add Cargo Deny policy for advisories, licenses, and sources; the remaining h2 advisory is narrowly ignored because Actix HTTP 3.x still requires h2 0.3.x.
• Add CI gates for checking, formatting, Clippy, tests, doctests, documentation, MSRV, dependency policy, and coverage.
• Remove obsolete Sonar configuration and avoid starting an unused PostgreSQL service in the general test job.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo test --workspace --all-features --locked --no-fail-fast
• cargo test --doc --workspace --all-features --locked
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps --locked
• cargo +1.88.0 check --workspace --all-targets --locked
• cargo deny check

Cargo Deny passes with duplicate-version and unmatched-license warnings. cargo nextest is configured in CI; it was not available in the local environment, so the equivalent locked workspace test suite was run locally.

Related issue: CHE-6

[linear-code]

CHE-6