enhance(citation): render citations server-side (#39373)

Replace citation-js with a Go port of ruby-cff, which GitHub uses for
"Cite this repository", rendering APA and BibTeX server-side and
dropping about 770KB of JS. Output matches GitHub on 1568 of 1571
real-world files, the rest are improvements over GitHub.

- `CITATION.cff` wins over `CITATION.bib`, matched case-insensitively
and through symlinks
- `CITATION.bib` is offered as-is, without APA

Signed-off-by: silverwind <me@silverwind.io>

modules/citation/cff_test.go

@@ -0,0 +1,137 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package citation
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFormatCFF(t *testing.T) {
+	cases := []struct{ cff, apa, bibtex string }{
+		{
+			cff: `title: Overridden
+message: &mit MIT
+authors: [{family-names: Haines, given-names: Robert}, {name: "The {curly_braces} Collective"}]
+title: "Software that uses the following symbols: &, %, $, #"
+version: 2024-01-16
+license: [*mit, Apache-2.0]
+date-released: 2024-01-16
+`,
+			apa: "Haines, R., & The {curly_braces} Collective. (2024). Software that uses the following symbols: &, %, $, # (Version 2024-01-16) [Computer software]",
+			bibtex: `@software{Haines_Software_that_uses_2024,
+author = {Haines, Robert and {The \{curly\_braces\} Collective}},
+license = {["MIT", "Apache-2.0"]},
+month = jan,
+title = {{Software that uses the following symbols: \&, \%, \$, \#}},
+version = {2024-01-16},
+year = {2024}
+}`,
+		},
+		{
+			cff: `authors:
+  - family-names: Smith
+    given-names: Arfon M.
+title: "Software citation principles"
+license: MIT
+preferred-citation:
+  authors:
+    - family-names: Smith
+      given-names: A. M.
+    - name: "FORCE11 Software Citation Working Group"
+  doi: "10.7717/peerj-cs.86"
+  journal: "PeerJ Computer Science"
+  month: 9
+  start: e86
+  title: "Software citation principles"
+  type: article
+  volume: 2
+  issue: 123
+  year: 2016
+  publisher:
+    - name: The Open Journal
+`,
+			apa: "Smith, A. M., & FORCE11 Software Citation Working Group. (2016). Software citation principles. PeerJ Computer Science, 2(123), e86. https://doi.org/10.7717/peerj-cs.86",
+			bibtex: `@article{Smith_Software_citation_principles_2016,
+author = {Smith, A. M. and {FORCE11 Software Citation Working Group}},
+doi = {10.7717/peerj-cs.86},
+journal = {PeerJ Computer Science},
+month = sep,
+number = {123},
+pages = {e86},
+title = {{Software citation principles}},
+volume = {2},
+year = {2016}
+}`,
+		},
+		{
+			cff: `preferred-citation:
+  type: conference-paper
+  version: 1.10
+  title: "Über tools"
+  authors:
+    - family-names: Ørsted
+      given-names: ǰan christian
+      name-particle: van
+      name-suffix: Jr.
+  collection-title: "Proceedings of X & Y"
+  conference:
+    name: "Conf_2020"
+    city: Berlin
+    country: DE
+    date-start: 30.06.2020
+    date-end: 2020-07-02
+  start: 10
+  end: 20
+  editors:
+    - affiliation: Press
+  editors-series:
+    - family-names: Editor
+      given-names: Eve
+`,
+			apa: "van Ørsted, J̌. C., Jr. (2020, June 30–July 2). Über tools (Version 1.10) [Conference paper]. Proceedings of X & Y, 10–20",
+			bibtex: `@inproceedings{van_Orsted_Uber_tools_2020,
+address = {Berlin, DE},
+author = {van Ørsted, Jr., ǰan christian},
+booktitle = {Proceedings of X \& Y},
+month = jun,
+pages = {10--20},
+series = {Conf\_2020},
+title = {{Über tools}},
+year = {2020}
+}`,
+		},
+		{
+			cff: `thesis: &thesis {type: phdthesis, title: Thesis}
+preferred-citation:
+  <<: *thesis
+  authors:
+    - family-names: Nguyễn
+      given-names: Sam
+      affiliation: "Uni_A"
+  institution: {city: Hanoi}
+  license: &loop [*loop]
+  status: in-press
+  notes: A note
+`,
+			apa: "Nguyễn, S. (in press). Thesis. [Doctoral dissertation, ]",
+			bibtex: `@phdthesis{Nguyn_Thesis_in_press,
+author = {Nguyễn, Sam},
+note = {A note},
+title = {{Thesis}},
+year = {in press}
+}`,
+		},
+		{cff: "title: No authors\nauthor:\n  - name: Typo\n"},
+	}
+	for _, tc := range cases {
+		t.Run("", func(t *testing.T) {
+			t.Parallel()
+			apa, bibtex := FormatCFF(tc.cff)
+			assert.Equal(t, tc.apa, apa)
+			assert.Equal(t, tc.bibtex, bibtex)
+		})
+	}
+}