refactor(line): use official LINE Bot SDK v8

[ex-takashima]

📝 Description

Replace hand-rolled HTTP/HMAC/JSON code (~270 lines) in the LINE channel with the official line-bot-sdk-go v8, reducing maintenance burden and eliminating potential bugs in signature verification, request construction, and response parsing.

This continues the work from #500 by @xiaket, which has been inactive for over a month. With @yinwm's encouragement (#500 (review)), I've rebased onto current main and addressed all review feedback.

🗣️ Type of Change

• 🐞 Bug fix
• ✨ New feature
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written

Changes

• Replace bytes/crypto/encoding/json/io imports with line-bot-sdk-go/v8
• Use webhook.ParseRequest for body reading + signature verification
• Use messaging_api.MessagingApiAPI for ReplyMessage/PushMessage/ShowLoadingAnimation/GetBotInfo
• Type-switch on webhook.MessageEvent message types (TextMessageContent, ImageMessageContent, etc.)
• Type-switch on webhook.SourceInterface (UserSource/GroupSource/RoomSource)
• Type-switch on webhook.Mentionee (UserMentionee/AllMentionee)

Net change: +174 / −295 lines in pkg/channels/line/line.go.

Review feedback addressed (from #500)

Issue	Reviewer	Fix
Context lost in API calls	@yinwm	Use WithContext(ctx) on all SDK calls
Variable shadowing of isMentioned	@yinwm	Declared at function scope, set inside switch
reflect-based message ID extraction	@nikolasdehor / @yinwm	Use type switch + msg.Id directly
Body size limit conflict with #1413	@yinwm	Wrap r.Body with http.MaxBytesReader before webhook.ParseRequest; check for *http.MaxBytesError to return 413
Mention detection	—	Use mentionee.IsSelf (new SDK field) for cleaner bot detection

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• All existing tests pass without modification (go test ./pkg/channels/line/...)
• Rebased onto current main

Test plan

• go build ./pkg/channels/line/
• go test ./pkg/channels/line/ -v — 5/5 tests pass
• Cross-compile for linux/arm64 works
• Verify on real LINE bot in staging

Notes

• This PR is independent from [refactor]use line bot sdk #500 (a fresh branch off current main). If [refactor]use line bot sdk #500 is preferred, I'm happy to push this work there instead — please let me know.
• Credit to @xiaket for the original SDK direction.