ci(deploy): trigger backend deploys on push to main, not develop

[tyrellcurry]

Now that main is the default/release branch and Vercel deploys from it, the backend's deploy-backend.yml was still only watching develop, so a release (a develop -> main PR) would update the frontend and GitHub's default branch but never actually redeploy the backend.

• deploy-backend.yml: trigger on push to main instead of develop.
• Root and backend READMEs: updated the deploy description to match (day-to-day work on develop, a develop -> main PR is a release).
• Small stray addition to backend/.gitignore (.vercel), harmless, defensive since backend has no Vercel project but doesn't hurt to ignore it there too.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
invoice-app	Ready	Preview	Aug 8, 2026 3:30am

[Copilot]

Pull request overview

Updates the backend deployment workflow and related documentation to reflect main as the release branch, ensuring backend deploys occur when releases land on main (while day-to-day work continues on develop).

Changes:

• Switch backend deploy GitHub Actions trigger from develop to main.
• Update root and backend deployment docs to match the release flow (develop → PR into main).
• Add .vercel to backend/.gitignore as a defensive ignore.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Updates deployment narrative to state releases deploy from main (with develop as day-to-day).
backend/README.md	Aligns backend deployment documentation with main as the release branch.
backend/.gitignore	Ignores .vercel in the backend directory.
.github/workflows/deploy-backend.yml	Triggers backend deploy workflow on pushes to main instead of develop.

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