enhance(actions): add OIDC workload identity support (#39052)

Adds OpenID Connect workload identity to Gitea Actions: jobs declaring
`id-token: write` can request short-lived tokens to authenticate to
cloud providers and other services without stored secrets. Tokens follow
GitHub's format, so clients like `@actions/core` and existing trust
policies carry over.

- Issuer at `/api/actions/oidc` with discovery and JWKS, signed with a
dedicated RS256 key
- GitHub's claims, including `workflow_ref`, `job_workflow_ref` and the
immutable `repo:owner@id/repo@id:ref:...` subject
- `id-token: write` or `write-all` grant it, default permissions and
fork pull requests never get it, owner and repository maximum
permissions can disable it
- Reusable workflows record the exact ref they were loaded from, and
dispatch rejects refs that only resolve through a same-named tag
- Generated signing keys are written atomically, and generated ES384 and
ES512 OAuth2 keys use their own curves instead of P-256
- `github.ref_type` is `branch` for pull request runs, like on GitHub

Fixes https://github.com/go-gitea/gitea/issues/26383
Fixes https://github.com/go-gitea/gitea/issues/33681
Docs: https://gitea.com/gitea/docs/pulls/526
Assisted-by: ChatGPT:gpt-5.6-sol, Claude Code:claude-opus-5-5
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

options/locale/locale_en-US.json

@@ -4002,6 +4002,8 @@
   "actions.general.token_permissions.maximum.description": "Actions job's effective permissions will be limited by the maximum permissions.",
   "actions.general.token_permissions.fork_pr_note": "If a job is started by a pull request from a fork, its effective permissions won't exceed the read-only permissions.",
   "actions.general.token_permissions.customize_max_permissions": "Customize maximum permissions",
+  "actions.general.token_permissions.id_token": "OpenID Connect token",
+  "actions.general.token_permissions.id_token.description": "Allow jobs declaring \"id-token: write\" to request short-lived OpenID Connect tokens for authenticating to external services.",
   "actions.general.cross_repo": "Cross-Repository Access",
   "actions.general.cross_repo_desc": "Allow the selected repositories to be accessed (read-only) by all the repositories in this owner with GITEA_TOKEN when running Actions jobs.",
   "actions.general.cross_repo_selected": "Selected repositories",