Support native Windows services and systemd lifecycle notifications

[lovasoa]

SQLPage currently cannot run directly under the Windows Service Control Manager, and systemd considers it started before database initialization or migrations finish. This adds native Windows service mode (--service NAME --web-root <absolute path>) and systemd readiness/stopping notifications, with one graceful shutdown path for service controls and Unix signals.

• Report readiness after database initialization, migrations, and HTTP listener/worker startup. Drain active requests for up to 30 seconds, close the database, and flush telemetry before exiting. Startup failures remain failures to the supervisor.
• Set the Windows service working directory explicitly and report SCM lifecycle states and failure exit codes. Write Application Event Log records through a background thread with a persistent native handle. A bounded queue holds 256 records of at most 16 KiB; overload drops records with a warning summary, and shutdown flushes accepted records before reporting STOPPED.
• Update the systemd unit to Type=notify, bounded startup/stop timeouts, journal logging, and restart-on-failure. Add installation and operation guides for both platforms.

Validation:

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test: 329 local tests passed, including real-process readiness, in-flight request draining for SIGTERM/SIGINT/SIGQUIT, bind failure, termination during database startup, and logging queue behavior.
• systemd-analyze verify on a temporary copy of the unit with ExecStart pointing to the locally built binary.
• CI at e7059079: Windows passed 331 Rust tests plus a real SCM integration test covering failed startup, readiness, active-request draining, clean stop, log flushing, and restart. Linux linting, all six database test configurations, and Playwright passed.

Impact measured on Linux x86_64, Rust 1.98.1, the shipping superoptimized profile (fat LTO), static ODBC, and identical frontend assets:

• Release binary: 34,144,688 to 34,250,880 bytes, an increase of 106,192 bytes (104 KiB, 0.31%).
• Root-crate rebuild with dependencies already built: baseline 402.62 seconds, current PR 396.23 seconds. These single samples do not establish a speedup or regression. An earlier comparison against a much faster first baseline was not reproducible and does not support a 40% compile-time increase.
• Five interleaved 300,000-request samples with routine logging disabled: median 107.8k vs 107.6k requests/second, with overlapping ranges; median RSS 22.27 vs 22.25 MiB. No meaningful runtime regression was demonstrated in this workload.
• Linux adds only sd-notify (0.19 seconds to compile in the initial measurement), with no native libsystemd dependency. Tokio signals were already enabled transitively. Windows adds windows-service and Event Log API features in the existing windows-sys dependency.

Windows release size, compile-time deltas, and throughput have not been measured. Its background logger adds one thread, a bounded queue, and formatting/enqueue work on producers; native Event Log calls run on the writer thread. It can drop log records under sustained overload, as documented.