Refactor HTTP channels to share transport logic

[mic1on]

Summary

Refactor HTTP-backed notification channels to share sync and async transport logic without changing the public API.

• Add HttpChannel as the shared transport base for HTTP providers.
• Migrate Bark, Chanify, Ding, Feishu, WeChat, Ntfy, PushDeer, and PushOver onto the shared base.
• Preserve channel-specific payload builders, dynamic credential resolution, provider response validation, and success logs.
• Add guard tests for unsupported HTTP method and payload kind configuration.
• Keep Email, Console, and Publisher retry behavior unchanged.

Closes #59

Design

• Spec: docs/superpowers/specs/2026-07-11-http-channel-transport-design.md
• Plan: docs/superpowers/plans/2026-07-11-http-channel-transport.md

Verification

• make lint
• make test (129 passed)
• make coverage (129 passed, total coverage 97%)
• uv build
• uv pip check