Support dynamic channel credentials

[mic1on]

Summary

• allow channel credential fields to be static values or zero-argument callables
• resolve callable credentials when building provider URLs or request payloads
• cover callable credentials for Bark, Chanify, Ding, Feishu, WeChat, Ntfy, PushDeer, PushOver, and Notify.from_settings
• document dynamic credential usage and clarify that provider-specific refresh remains application-owned

Closes #13

Verification

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