feat(api): add q parameter to list branches API for server-side filtering (#37982)

The GET /repos/{owner}/{repo}/branches endpoint currently has no way to
filter branches by name server-side, forcing API consumers to paginate
through all branches and filter client-side.

The UI already supports branch search (added in
[#27055](https://github.com/go-gitea/gitea/pull/27055)). The underlying
DB layer has a Keyword field on FindBranchOptions in
models/git/branch_list.go that does a LIKE %keyword% SQL filter, it just
wasn't wired up to the API handler.

This PR exposes a ?q= query parameter on the endpoint that maps to
FindBranchOptions.Keyword.

Example:

```GET /repos/owner/repo/branches?q=feature ```
Closes #37981

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/swagger/v1_json.tmpl

@@ -7158,6 +7158,12 @@
             "description": "page size of results",
             "name": "limit",
             "in": "query"
+          },
+          {
+            "type": "string",
+            "description": "branch name substring to filter by",
+            "name": "q",
+            "in": "query"
           }
         ],
         "responses": {

templates/swagger/v1_openapi3_json.tmpl

@@ -18224,6 +18224,14 @@
             "schema": {
               "type": "integer"
             }
+          },
+          {
+            "description": "branch name substring to filter by",
+            "in": "query",
+            "name": "q",
+            "schema": {
+              "type": "string"
+            }
           }
         ],
         "responses": {