feat(branch): créer la branche depuis la carte (issue #3)

slugify + branch_name_for (issue-<n>-<slug>, reconnu par #1/#2);
client create_branch; POST /api/issues/{o}/{r}/{n}/branch (nom
libre validé, idempotent si existante); la carte passe en En
cours, journalisée source branch, étiquette synchronisée;
formulaire dans la section Code lié du panneau détail.

app/gitea_client.py

@@ -341,6 +341,19 @@ class GiteaClient:
         self._cache_clear()
         return r.json()
 
+    async def create_branch(
+        self, owner: str, repo: str, new_branch: str, old_branch: str
+    ) -> dict:
+        """Create a branch from another one (issue #3)."""
+        return await self._request(
+            "POST",
+            f"/repos/{owner}/{repo}/branches",
+            json={
+                "new_branch_name": new_branch,
+                "old_branch_name": old_branch,
+            },
+        )
+
     async def list_branches(self, owner: str, repo: str) -> list[dict]:
         """All branches of a repo (code links, issue #1)."""
         return await self._get_all_pages(f"/repos/{owner}/{repo}/branches")