fix(webhook): remove legacy webhook endpoint

[tri-pti]

Description

Closes #7876

This PR removes the legacy /api/webhook endpoint and its associated test since /api/webhooks/github is the only supported GitHub webhook endpoint.

It also updates the self-hosting documentation to clearly document the correct webhook configuration.

Changes

• Removed app/api/webhook/route.ts
• Removed app/api/webhook/route.test.ts
• Added a new section in docs/self_hosting.md documenting:
  • GITHUB_WEBHOOK_SECRET
  • the correct webhook endpoint (/api/webhooks/github)
  • the required GitHub webhook configuration
  • that this is the only supported webhook endpoint

Pillar

• 🛠️ Other (Bug fix, refactoring, docs)

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally.
• I have run npm run format and npm run lint locally and resolved all errors.
• My commits follow the Conventional Commits format.
• README.md update not required.
• I have starred the repo.
• I have made sure that I have only one commit in this PR.
• The change does not affect SVG rendering.
• (Recommended) I joined the CommitPulse Discord community.

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[github-actions]

📦 Next.js Bundle Size Report (Gzipped Sizes)

Asset	PR Size	Base Size	Difference	Status
static/chunks/app/api/webhook/route-[hash].js	-	0.24 KB	-0.24 KB (-100.00%)	🗑️ Deleted

📊 Summary of Totals

Category	PR Size	Base Size	Difference
Total JS	3916.86 KB	3917.41 KB	-0.54 KB (-0.01%)
Total CSS	325.86 KB	325.86 KB	0 B

[Aamod007]

Hello! Thanks for removing the legacy webhook endpoint. It's always great to clean up dead code and reduce our attack surface. Removing unused routes keeps the API surface lean. CI checks are looking green!

Labels assigned:

• \level:beginner: Removing a legacy endpoint is straightforward.
• \quality:clean: Good cleanup task.
• \ ype:refactor: Code cleanup/removal.