Fix the swagger update cron job (#481)

The scheduled `update swagger files` job has failed on **every** run since it was added (I checked the run list on gitea.com, ~250 consecutive failures). Two reasons:

1. Upstream moved the file: `go-gitea/gitea` main no longer has `templates/swagger/v1_json.tmpl`, it is now `templates/swagger/v1-swagger.generated.json`, and the placeholders changed from `{{.SwaggerAppVer}}` / `{{.SwaggerAppSubUrl}}` to `0.0.0+GITEA-API-APP-VERSION` / `/GITEA-API-APP-SUBURL/api/v1`. Because the script used `curl --silent` without `--fail`, the 404 body was written into `static/swagger-latest.json`.
2. `main` is protected (`required_approvals=1`), so the job's `git push` to main could never succeed.

Changes:

- `update_api_docs.sh`: `set -euo pipefail`, download with `--fail` (a broken download now aborts instead of committing garbage), try the new generated json first and fall back to the old template for released tags, handle the 1.28+/1.24+/<1.24 placeholder variants, and add a `--latest-only` flag.
- `Makefile`: new `update-api-docs-latest` target for the cron job; `clean` no longer deletes the committed `static/swagger-*.json` files (deleting them breaks `make build`).
- `.gitea/workflows/update_swagger.yaml`: every 12h (plus `workflow_dispatch`) it regenerates only `static/swagger-latest.json`, verifies no placeholder is left and the file is valid JSON, force pushes to `bot/update-swagger-latest` and opens a pull request via the API (HTTP 409 = a PR is already open, it just now points at the new commit). It skips entirely when the bot branch already carries the same file, so no PR churn.
- `README.md`: document the two make targets and the automation.

`secrets.DEPLOY_TOKEN` must belong to a real user with `write:repository` so the PR checks are triggered; the job fails with a clear message if it is empty.

Tested locally in a scratch clone: `make update-api-docs-latest` produces a valid `dev` spec with no placeholders left, `make update-api-docs` still regenerates all released versions, and the workflow shell logic was simulated against a local bare repo for all three paths (open PR / bot branch already up to date / main already up to date), including a shallow clone to confirm `fetch-depth: 1` force pushes work.

Fixes #454

Reviewed-on: https://gitea.com/gitea/docs/pulls/481
Reviewed-by: techknowlogick <[email protected]>

Makefile

@@ -41,18 +41,17 @@ serve: prepare-docs
 serve-zh: prepare-docs
 	pnpm run start -- --locale zh-cn
 
+# static/swagger-*.json are committed files, use update-api-docs to refresh them
 .PHONY: clean
 clean:
 	rm -rf .tmp
 	rm -rf static/_*
-	rm -rf static/swagger-latest.json
-	rm -rf static/swagger-19.json
-	rm -rf static/swagger-20.json
-	rm -rf static/swagger-21.json
-	rm -rf static/swagger-22.json
-	rm -rf static/swagger-23.json
-	rm -rf static/swagger-24.json
 
 .PHONY: update-api-docs
 update-api-docs:
 	./update_api_docs.sh
+
+# only refresh static/swagger-latest.json (used by the update swagger cron job)
+.PHONY: update-api-docs-latest
+update-api-docs-latest:
+	./update_api_docs.sh --latest-only