Add LINE webhook integration for community feed notifications

[ashryanbeats]

Context

The Community Feed Notifier currently posts new community feed items through configured destinations such as Discord/generic webhooks. A hackathon PR explored adding LINE integration, but we should not land that directly yet.

Goal

Add LINE webhook support as a deliberate follow-up after the shared feed reader and notifier test coverage are stable.

Scope

• Define the intended LINE destination configuration and required repository secrets.
• Add a LINE delivery adapter without changing feed polling, gist/state behavior, or item identity semantics.
• Cover payload construction with tests.
• Update README setup instructions.

Notes

• Treat the existing LINE PR/prototype as reference material, not as an implementation to merge wholesale.
• Keep this separate from the current notifier rearchitecture work.