fix(config): reject snapshot retention shorter than interval

[UniversePeak]

Summary

Reject configurations where snapshot retention is shorter than the snapshot interval.

Problem

A configuration with snapshot.interval greater than snapshot.retention currently starts successfully even though retention cannot preserve a full snapshot interval. This makes the retention setting ineffective and provides no startup signal.

Solution

Validate the relationship for both global and per-database snapshot settings, returning a dedicated configuration validation error.

Scope

In scope:

• Global snapshot.retention versus snapshot.interval validation
• Per-database snapshot relationship validation
• Focused parser regression tests

Not in scope:

• Runtime retention behavior
• Changes to existing positive-duration validation

Test Plan

• go test ./cmd/litestream -run 'TestConfig' -count=1
• go vet ./cmd/litestream

Both commands pass locally. The full package suite has unrelated Windows-path and file-locking failures in this checkout.

Related

• Fixes Nothing complains if I set retention interval lower than snapshot interval #1451

[UniversePeak]

This PR has been open for over a week with all reported checks passing, including the focused config tests, lint, and cross-platform build checks. I’m following up on the review status when you have a chance. The change is limited to validating global and per-database snapshot retention against their intervals, with no runtime retention behavior changes.