feat: add Gateway API support (#1073)

Add full Gateway API support for exposing Gitea via HTTPRoute, TCPRoute, BackendTLSPolicy, and ClientSettingsPolicy resources.

New templates:
- `httpRoute.yaml` — renders an HTTPRoute with configurable
  parentRefs, hostnames, and rules (defaults to PathPrefix `/`)
- `tcpRoute.yaml` — renders a TCPRoute for SSH traffic
- `backendTLSPolicy.yaml` — renders a BackendTLSPolicy for
  encrypted backend connections with required validation config
- `clientSettingsPolicy.yaml` — renders an NGINX Gateway Fabric
  ClientSettingsPolicy to raise the request body size limit

Infrastructure:
- `gatewayAPI.enabled` global toggle gates all resources
- Resources grouped under `gatewayAPI.core.*` and `gatewayAPI.nginx.*`
- Helper templates extracted into dedicated `_*.tpl` files
- Service name helpers (`gitea.service.http.name`, `gitea.service.ssh.name`)
  extracted into `_services.tpl`; service templates renamed to camelCase
- `ROOT_URL`, `DOMAIN`, and `SSH_DOMAIN` auto-resolve from
  `httpRoute.hostnames[0]`; `httpRoute.tls` switches to `https`

Documentation:
- New `docs/gateway-api.md` with topology examples, BackendTLSPolicy
  setup, sectionName guidance, SSH considerations, and NGINX body
  size limit configuration
- `.github/copilot-instructions.md` with project conventions
- README parameter table auto-generated via `make readme`

Tests:
- Helm unit tests for all four new resource templates
- Config tests for hostname/TLS resolution from Gateway API values

Co-authored-by: Todd Marimon <toddmarimon@gmail.com>

templates/gitea/backendTLSPolicy.yaml

@@ -0,0 +1,30 @@
+{{- if eq (include "gitea.backendTLSPolicy.enabled" .) "true" -}}
+{{- if not (keys .Values.gatewayAPI.core.backendTLSPolicy.validation) }}
+{{- fail "gatewayAPI.core.backendTLSPolicy.validation is required" }}
+{{- end }}
+---
+apiVersion: gateway.networking.k8s.io/v1
+kind: BackendTLSPolicy
+metadata:
+  {{- with (include "gitea.backendTLSPolicy.annotations" .) }}
+  annotations:
+    {{- . | nindent 4 }}
+  {{- end }}
+  {{- with (include "gitea.backendTLSPolicy.labels" .) }}
+  labels:
+    {{- . | nindent 4 }}
+  {{- end }}
+  name: {{ include "gitea.fullname" . }}
+  namespace: {{ .Values.namespace | default .Release.Namespace }}
+spec:
+  targetRefs:
+    {{- if .Values.gatewayAPI.core.backendTLSPolicy.targetRefs }}
+    {{- toYaml .Values.gatewayAPI.core.backendTLSPolicy.targetRefs | nindent 4 }}
+    {{- else }}
+    - group: ""
+      kind: Service
+      name: {{ include "gitea.service.http.name" . }}
+    {{- end }}
+  validation:
+    {{- toYaml .Values.gatewayAPI.core.backendTLSPolicy.validation | nindent 4 }}
+{{- end }}