feat(0.35.0): jalons en écriture depuis la page Projet (issue #26)

- Clients : create_milestone / update_milestone (cache
  invalidé) ; GitHub hérite des mêmes endpoints.
- API Kanbante : création, modification (titre/échéance,
  échéance vidée = retirée), fermeture ; CSRF, validation
  de date avant toute écriture, erreurs forge en 502.
- Page Projet : formulaire 'Nouveau jalon' + édition/
  fermeture par jalon (project.js versionné).
- Échéance à midi UTC : à minuit, le fuseau de l'instance
  Gitea faisait reculer la date affichée d'un jour (réel).
- 291 tests verts (5 nouveaux, palier 24) ; cycle réel
  complet sur Projets/kanbante (jalons smoke #37/#38 créés,
  dates vérifiées, fermés).

tests/conftest.py

@@ -359,15 +359,64 @@ class FakeGitea:
         else:
             self.starred.discard(f"{owner}/{repo}")
 
-    async def get_milestones(self, owner, repo, state="open"):
-        return [
+    @property
+    def _milestones(self):
+        return self.__dict__.setdefault(
+            "milestones_store",
             {
-                "id": 1, "title": "Sprint 1",
-                "due_on": "2026-10-20T00:00:00Z",
-                "open_issues": 1, "closed_issues": 1,
-            }
+                "Projets/demo": [
+                    {
+                        "id": 1, "title": "Sprint 1",
+                        "description": "",
+                        "due_on": "2026-10-20T00:00:00Z",
+                        "state": "open",
+                        "open_issues": 1, "closed_issues": 1,
+                    }
+                ]
+            },
+        )
+
+    async def get_milestones(self, owner, repo, state="open"):
+        items = self._milestones.get(f"{owner}/{repo}", [])
+        return [
+            dict(m) for m in items
+            if state == "all" or m.get("state", "open") == state
         ]
 
+    async def create_milestone(
+        self, owner, repo, title, description="", due_on=None
+    ):
+        store = self._milestones.setdefault(f"{owner}/{repo}", [])
+        milestone = {
+            "id": 100 + len(store),
+            "title": title,
+            "description": description,
+            "due_on": due_on,
+            "state": "open",
+            "open_issues": 0, "closed_issues": 0,
+        }
+        store.append(milestone)
+        return dict(milestone)
+
+    async def update_milestone(
+        self, owner, repo, milestone_id, title=None, description=None,
+        due_on=None, clear_due=False, state=None,
+    ):
+        for m in self._milestones.get(f"{owner}/{repo}", []):
+            if m["id"] == milestone_id:
+                if title is not None:
+                    m["title"] = title
+                if description is not None:
+                    m["description"] = description
+                if clear_due:
+                    m["due_on"] = None
+                elif due_on:
+                    m["due_on"] = due_on
+                if state is not None:
+                    m["state"] = state
+                return dict(m)
+        raise AssertionError("jalon introuvable dans le faux")
+
     async def create_issue(self, owner, repo, title, body=""):
         self._next_number += 1
         issue = _issue(self._next_number, title)