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>

unittests/helm/config/server-section_domain.yaml

@@ -103,3 +103,56 @@ tests:
         matchRegex:
           path: stringData.server
           pattern: \nROOT_URL=https://route.example.com
+
+  ################################################
+
+  - it: "[HTTPRoute enabled] uses first hostname for DOMAIN|SSH_DOMAIN|ROOT_URL"
+    template: templates/gitea/config.yaml
+    set:
+      ingress:
+        hosts: []
+      gatewayAPI:
+        enabled: true
+        core:
+          httpRoute:
+            enabled: true
+            hostnames:
+              - gw.example.com
+            parentRefs:
+              - name: shared-gateway
+    asserts:
+      - documentIndex: 0
+        matchRegex:
+          path: stringData.server
+          pattern: \nDOMAIN=gw.example.com
+      - documentIndex: 0
+        matchRegex:
+          path: stringData.server
+          pattern: \nSSH_DOMAIN=gw.example.com
+      - documentIndex: 0
+        matchRegex:
+          path: stringData.server
+          pattern: \nROOT_URL=http://gw.example.com
+
+  ################################################
+
+  - it: "[HTTPRoute tls] switches ROOT_URL to https"
+    template: templates/gitea/config.yaml
+    set:
+      ingress:
+        hosts: []
+      gatewayAPI:
+        enabled: true
+        core:
+          httpRoute:
+            enabled: true
+            tls: true
+            hostnames:
+              - gw.example.com
+            parentRefs:
+              - name: shared-gateway
+    asserts:
+      - documentIndex: 0
+        matchRegex:
+          path: stringData.server
+          pattern: \nROOT_URL=https://gw.example.com