feat(client): add GiteaClient with auth and pagination (fixes #1)

- GiteaClient with requests.Session and token auth header
- _get_paginated for automatic pagination (limit=50)
- get_repos, get_latest_release (None on 404), get_milestones
- 9 unit tests with mocked requests.Session
- Fix setuptools build backend in pyproject.toml

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/gitea_dashboard/client.py

@@ -0,0 +1,73 @@
+"""Client API Gitea en lecture seule."""
+
+from __future__ import annotations
+
+import requests
+
+
+class GiteaClient:
+    """Client API Gitea en lecture seule.
+
+    Utilise requests.Session pour reutiliser les connexions HTTP.
+    Authentification via header Authorization: token <TOKEN>.
+    """
+
+    _PAGE_LIMIT = 50
+
+    def __init__(self, base_url: str, token: str) -> None:
+        """Initialise le client avec l'URL de base et le token API."""
+        self.base_url = base_url.rstrip("/")
+        self.session = requests.Session()
+        self.session.headers["Authorization"] = f"token {token}"
+
+    def _get_paginated(self, endpoint: str, params: dict | None = None) -> list[dict]:
+        """Requete GET avec pagination automatique.
+
+        Boucle tant que len(page) == limit (50).
+        """
+        all_items: list[dict] = []
+        page = 1
+        merged_params = dict(params) if params else {}
+
+        while True:
+            merged_params["limit"] = self._PAGE_LIMIT
+            merged_params["page"] = page
+            url = f"{self.base_url}{endpoint}"
+            resp = self.session.get(url, params=merged_params)
+            resp.raise_for_status()
+            items = resp.json()
+            all_items.extend(items)
+            if len(items) < self._PAGE_LIMIT:
+                break
+            page += 1
+
+        return all_items
+
+    def get_repos(self) -> list[dict]:
+        """Retourne tous les repos de l'utilisateur (pagination automatique).
+
+        Endpoint: GET /api/v1/user/repos
+        """
+        return self._get_paginated("/api/v1/user/repos")
+
+    def get_latest_release(self, owner: str, repo: str) -> dict | None:
+        """Retourne la derniere release du repo, ou None si aucune.
+
+        Endpoint: GET /api/v1/repos/{owner}/{repo}/releases/latest
+        Gere HTTP 404 en retournant None.
+        """
+        url = f"{self.base_url}/api/v1/repos/{owner}/{repo}/releases/latest"
+        resp = self.session.get(url)
+        if resp.status_code == 404:
+            return None
+        return resp.json()
+
+    def get_milestones(self, owner: str, repo: str) -> list[dict]:
+        """Retourne les milestones ouvertes du repo.
+
+        Endpoint: GET /api/v1/repos/{owner}/{repo}/milestones?state=open
+        """
+        return self._get_paginated(
+            f"/api/v1/repos/{owner}/{repo}/milestones",
+            params={"state": "open"},
+        )