Prepare 0.4.0 release

[mic1on]

Summary

Prepare the 0.4.0 release after the recent feature, reliability, and test coverage work.

• Bump package metadata from 0.3.5 to 0.4.0.
• Add CHANGELOG.md with 0.4.0 release notes.
• Sync uv.lock with the new local package version.

Closes #55

Release Notes

• Dynamic channel credentials for runtime token/topic/user resolution.
• CI coverage enforcement at 95% minimum total coverage.
• Explicit channel registry lookup for Notify.from_settings.
• Fixes for title-less Chanify/WeChat payloads, Email async logging, URL redaction, and bool numeric config validation.

Verification

• make lint
• make coverage (126 passed, total coverage 97%)
• uv build
• uv pip check