feat(web): support ssh owner repo clone flow

- Add SSH-only owner/repo clone path for git.commumedia.org
- Preflight remote repository existence with git ls-remote before cloning
- Keep advanced URL paste fallback and blank repository creation
- Add focused backend and frontend coverage plus docs updates

Quality gates: python -m py_compile, vitest run src/components/repositories-settings-tab.test.tsx, npm run typecheck

apps/api/src/api/git_repositories.py

@@ -89,6 +89,32 @@ def _get_repo_path(user_id: uuid.UUID, project_id: uuid.UUID, name: str) -> str:
     return os.path.join(base, str(user_id), str(project_id), f"{name}.git")
 
 
+def _build_provider_clone_url(owner: str, repo: str) -> str:
+    """Build the SSH clone URL for the fixed git provider."""
+    return f"git@git.commumedia.org:{owner}/{repo}.git"
+
+
+def _preflight_remote_repository(remote_url: str) -> None:
+    """Verify a remote repository is reachable before cloning."""
+    try:
+        result = subprocess.run(
+            ["git", "ls-remote", remote_url],
+            capture_output=True,
+            text=True,
+            timeout=60,
+        )
+    except subprocess.TimeoutExpired:
+        raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="remote repository check timed out")
+    except FileNotFoundError:
+        raise HTTPException(status_code=status.HTTP_500_INTERNAL_SERVER_ERROR, detail="git command not found")
+
+    if result.returncode != 0:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="repository not found or inaccessible",
+        )
+
+
 class GitRepositoryCreate(BaseModel):
     name: str
     remote_url: str | None = None
@@ -267,6 +293,9 @@ async def create_repository(
         if parse_result["base_url"]:
             remote_url = parse_result["base_url"]
 
+    if remote_url:
+        _preflight_remote_repository(remote_url)
+
     repo_path = _get_repo_path(user_id, project_id, data.name)
 
     # Ensure parent directory exists