Add debug API endpoint for mail template rendering

[Varsius]

Checklist:

• I updated the documentation to describe the semantical or interface changes I introduced.

[SuperSandro2000]

Suggested change

	Requires the `?template_type` query parameter.
	Requires the `template_type` query parameter.

[Varsius]

The ? was added to be consistent with the existing admin endpoints documentation

[SuperSandro2000]

Should we list available templates here? The user needs admin anyhow, so we don't leak anything, right?

[wagnerd3]

I think this current state would be sufficient for me right now.

[Copilot]

Pull request overview

Adds an admin-only debug endpoint to render configured mail templates using dummy data, primarily to help validate mail template rendering during development/testing.

Changes:

• Add GET /admin/mail/render endpoint that renders a chosen template type using dummy commitment data.
• Extend test setup with WithMailTemplates and add an API test covering auth and parameter validation.
• Vendor golang.org/x/net/html (and update go.mod/go.sum/vendor/modules.txt) to parse the rendered output.

Reviewed changes

Copilot reviewed 6 out of 23 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
internal/api/mail.go	Implements the new mail template render endpoint and HTML parse check.
internal/api/core.go	Registers the new route in the v1 API router.
internal/api/mail_test.go	Adds tests for permissions and request parameter validation.
internal/test/setup.go	Adds WithMailTemplates option to inject dummy mail templates for tests.
docs/users/api-spec-resources.md	Documents the new endpoint and its query parameter.
go.mod	Adds direct dependency on golang.org/x/net.
go.sum	Updates checksums for updated golang.org/x/* dependencies.
vendor/modules.txt	Reflects newly vendored golang.org/x/net and updated golang.org/x/sys.
vendor/golang.org/x/net/html/token.go	Vendored HTML tokenizer implementation.
vendor/golang.org/x/net/html/render.go	Vendored HTML renderer implementation.
vendor/golang.org/x/net/html/parse.go	Vendored HTML parser implementation.
vendor/golang.org/x/net/html/node.go	Vendored DOM node model implementation.
vendor/golang.org/x/net/html/iter.go	Vendored iterator helpers for node traversal (go build tag).
vendor/golang.org/x/net/html/foreign.go	Vendored foreign content parsing helpers (SVG/MathML).
vendor/golang.org/x/net/html/escape.go	Vendored HTML escaping/unescaping helpers.
vendor/golang.org/x/net/html/doctype.go	Vendored doctype parsing logic.
vendor/golang.org/x/net/html/doc.go	Vendored package documentation.
vendor/golang.org/x/net/html/const.go	Vendored parsing constants/helpers.
vendor/golang.org/x/net/html/atom/table.go	Vendored generated atom table for tags/attrs.
vendor/golang.org/x/net/html/atom/atom.go	Vendored atom lookup implementation.
vendor/golang.org/x/net/LICENSE	Vendored license for golang.org/x/net.
vendor/golang.org/x/net/PATENTS	Vendored patents grant for golang.org/x/net.

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[VoigtS]

I left a couple of notes.

[Varsius]

The tests are failing since with the test cases added in this PR, the total amount of tests exceeds the concurrency limit of postgres connections in our testing setup. Increasing the amount of max_connections = 100 in postgressql.conf should do the trick, this likely needs to be changed in g-m-m.

[SuperSandro2000]

It's in go-bits 😅 fix at sapcc/go-bits#292

[github-actions]

Merging this branch will increase overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/limes/internal/api	78.40% (+0.24%)	👍

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/limes/internal/api/core.go	80.00% (+0.15%)	810 (+6)	648 (+6)	162	👍
github.com/sapcc/limes/internal/api/mail.go	87.23% (+87.23%)	282 (+282)	246 (+246)	36 (+36)	🌟

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/sapcc/limes/internal/api/mail_test.go