ci: lint PR titles with commitlint (#37498)

## Summary
- Enforce **Conventional Commits** on PR titles (PRs are squash-merged,
so the PR title becomes the final commit message).
- Add a local `make lint-pr-title` target so contributors can validate
titles before pushing.

## Why
We squash-merge PRs, which means the final repository history is largely
shaped by **PR titles**. Enforcing a consistent Conventional Commits
format makes:
- **Release notes & changelogs easier to generate** (types like `feat` /
`fix` can be grouped automatically).
- **History easier to scan** (uniform structure, optional scopes,
explicit breaking changes via `!`).
- **Automation more reliable** (future tooling can infer category and
scope from the title).

## PR title format
```text
type(scope)!: subject
type: one of build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test
scope: optional (e.g. web, api, actions, repo, …)
!: optional, indicates a breaking change
subject: short, imperative, no trailing period
```

## Examples
```text
feat(web): add dark mode toggle
fix(api): avoid panic when repo is missing
chore(ci): lint PR titles with commitlint
refactor(templates): reduce duplication in repo list rendering
feat!: remove legacy OAuth endpoint
```

## Local testing
```text
make deps-frontend
make lint-pr-title PR_TITLE="feat(web): add dark mode toggle"
```

---------

Signed-off-by: Nicolas <bircni@icloud.com>
Co-authored-by: nb <nb@users.noreply.local>
Co-authored-by: GPT-5.2 <gpt-5.2@openai.com>

.github/pull_request_template.md

@@ -1,10 +1,11 @@
 <!-- start tips -->
 Please check the following:
 1. Make sure you are targeting the `main` branch, pull requests on release branches are only allowed for backports.
-2. Make sure you have read contributing guidelines: https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md .
-3. For documentations contribution, please go to https://gitea.com/gitea/docs
-4. Describe what your pull request does and which issue you're targeting (if any).
-5. It is recommended to enable "Allow edits by maintainers", so maintainers can help more easily.
-6. Your input here will be included in the commit message when this PR has been merged. If you don't want some content to be included, please separate them with a line like `---`.
-7. Delete all these tips before posting.
+2. Use a Conventional Commits PR title, for example `fix(repo): handle empty branch names`.
+3. Make sure you have read contributing guidelines: https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md .
+4. For documentations contribution, please go to https://gitea.com/gitea/docs
+5. Describe what your pull request does and which issue you're targeting (if any).
+6. It is recommended to enable "Allow edits by maintainers", so maintainers can help more easily.
+7. Your input here will be included in the commit message when this PR has been merged. If you don't want some content to be included, please separate them with a line like `---`.
+8. Delete all these tips before posting.
 <!-- end tips -->

.github/workflows/pull-pr-title.yml

@@ -0,0 +1,28 @@
+name: pr-title
+
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - reopened
+      - synchronize
+      - ready_for_review
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  lint-pr-title:
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - run: make lint-pr-title
+        env:
+          PR_TITLE: ${{ github.event.pull_request.title }}