chore(ci): tighten backend workflow path filters

[laipz8200]

Important

1. Make sure you have read our contribution guidelines
2. Ensure there is an associated issue and you have been assigned to it
3. This PR is part of [Refactor/Chore] Optimize GitHub Actions CI cost across backend, web, VDB, and Docker lanes #34212. It intentionally does not use Fixes #34212 because the umbrella issue will remain open for follow-up CI optimization PRs.

Summary

Part of #34212.

• replace broad backend docker/** triggers with narrower file filters for API, VDB, and migration lanes
• keep required check job names stable in main-ci
• include VDB test-path changes in the VDB lane filter

Screenshots

Before	After
N/A	N/A

Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint and make type-check (backend) and cd web && npx lint-staged (frontend) to appease the lint gods

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/main-ci.yml")' passed
• the backend trigger set was narrowed to explicit runtime inputs instead of a blanket docker/** match