feat: add branch_count to repository API (#35351) (#36743)

Description
This PR adds a branch_count field to the repository API response.
Currently, clients have to fetch all branches via /branches just to
determine the total number of branches. This addition brings Gitea
closer to parity with GitLab's API and improves efficiency for UI/CLI
clients that need this metric.

Linked Issue
Fixes #35351

Changes
API Structs: Added BranchCount field to Repository struct in
modules/structs/repo.go.

Database Logic: Implemented CountBranches in models/git/branch.go using
XORM for efficient counting.

Service Layer: Updated the ToRepo conversion logic in
services/convert/repository.go to populate the new field during API
serialisation.

Tests: Added a new unit test TestCountBranches in
models/git/branch_test.go to verify counts (including handling of
deleted branches).

Screenshots
<img width="196" height="121" alt="Screenshot 2026-02-24 at 21 41 07"
src="https://github.com/user-attachments/assets/cd023e92-f338-448b-9e49-0a5d54cc96c2"
/>

Testing
Manually verified the output using curl against a local Gitea instance.

Verified that adding a branch increments the count and deleting a branch
(soft-delete) decrements it.

Ran backend linting: make lint-backend (Passed).

Ran specific unit test: go test -v -tags "sqlite sqlite_unlock_notify"
./models/git -run TestCountBranches (Passed).

Co-authored-by: silverwind <me@silverwind.io>

templates/swagger/v1_json.tmpl

@@ -28128,6 +28128,11 @@
           "type": "string",
           "x-go-name": "AvatarURL"
         },
+        "branch_count": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "BranchCount"
+        },
         "clone_url": {
           "type": "string",
           "x-go-name": "CloneURL"