enhance: Migrate remaining gopkg.in/yaml.v3 usages to go.yaml.in/yaml/v4 (#37866)

### Description
Replaces all remaining direct `gopkg.in/yaml.v3` imports with
`go.yaml.in/yaml/v4` across models, modules, routers, services, and
integration tests. `gopkg.in/yaml.v3` moves from a direct to an indirect
dependency in `go.mod`.

#### API compatibility

The yaml.Node type, node.Kind/node.Content traversal style
(modules/markup/markdown/convertyaml.go), and the
UnmarshalYAML(*yaml.Node) interface signature
(modules/optional/serialization.go) are all preserved in v4 — no
call-site changes were required beyond the import path.

**Related:**
- https://github.com/go-gitea/gitea/pull/36564#issuecomment-4526536805

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

go.mod

@@ -113,7 +113,6 @@ require (
 	google.golang.org/grpc v1.81.1
 	google.golang.org/protobuf v1.36.11
 	gopkg.in/ini.v1 v1.67.2
-	gopkg.in/yaml.v3 v3.0.1
 	modernc.org/sqlite v1.50.1
 	mvdan.cc/xurls/v2 v2.6.0
 	strk.kbt.io/projects/go/libravatar v0.0.0-20260301104140-add494e31dab
@@ -282,6 +281,7 @@ require (
 	golang.org/x/tools v0.44.0 // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260401020348-3a24fdc17823 // indirect
 	gopkg.in/warnings.v0 v0.1.2 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 	modernc.org/libc v1.72.3 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect