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).

app/main.py

@@ -3147,7 +3147,10 @@ async def project_page(request: Request, owner: str, repo: str):
         est = forecast_service.estimate(opened, weekly)
         milestones.append(
             {
+                "id": m.get("id"),
                 "title": m.get("title", ""),
+                "description": m.get("description") or "",
+                "due_iso": due_date.isoformat() if due_date else "",
                 "total": total,
                 "closed": closed,
                 "open": opened,
@@ -4373,6 +4376,136 @@ async def my_issues(request: Request):
     )
 
 
+def _milestone_due_iso(due: str) -> tuple[str | None, bool]:
+    """Validate a YYYY-MM-DD form date -> (ISO instant, ok)."""
+    due = (due or "").strip()
+    if not due:
+        return None, True
+    try:
+        parsed = datetime.strptime(due, "%Y-%m-%d")
+    except ValueError:
+        return None, False
+    # Midi UTC, pas minuit : Gitea renvoie l'échéance dans le
+    # fuseau de son instance (Toronto = UTC-5) — à minuit UTC,
+    # la date affichée reculait d'un jour (attrapé en réel).
+    return parsed.strftime("%Y-%m-%dT12:00:00Z"), True
+
+
+@app.post("/api/repos/{owner}/{repo}/milestones")
+async def api_milestone_create(
+    request: Request,
+    owner: str,
+    repo: str,
+    title: str = Form(""),
+    description: str = Form(""),
+    due_on: str = Form(""),
+    csrf_token: str = Form(""),
+):
+    """Create a milestone on the forge (issue #26)."""
+    if not _csrf_valid(request, csrf_token):
+        return _csrf_error()
+    title = title.strip()
+    if not title or len(title) > 255:
+        return JSONResponse(
+            {"ok": False, "error": "Titre requis (255 caractères max)"},
+            status_code=400,
+        )
+    due_iso, ok = _milestone_due_iso(due_on)
+    if not ok:
+        return JSONResponse(
+            {"ok": False, "error": "Échéance invalide (format AAAA-MM-JJ)"},
+            status_code=400,
+        )
+    client, _cid, _ws = await _resolve_board_scope(request, owner, repo)
+    try:
+        milestone = await client.create_milestone(
+            owner, repo, title, description.strip(), due_iso
+        )
+    except httpx.HTTPStatusError as exc:
+        return JSONResponse(
+            {"ok": False, "error": f"La forge répond {exc.response.status_code}"},
+            status_code=502,
+        )
+    return JSONResponse(
+        {"ok": True, "milestone": {
+            "id": milestone.get("id"),
+            "title": milestone.get("title"),
+            "due_on": milestone.get("due_on"),
+        }},
+        status_code=201,
+    )
+
+
+@app.post("/api/repos/{owner}/{repo}/milestones/{milestone_id}/update")
+async def api_milestone_update(
+    request: Request,
+    owner: str,
+    repo: str,
+    milestone_id: int,
+    title: str = Form(""),
+    due_on: str = Form(""),
+    csrf_token: str = Form(""),
+):
+    """Edit a milestone's title / due date (issue #26).
+
+    An empty due date CLEARS the milestone's deadline.
+    """
+    if not _csrf_valid(request, csrf_token):
+        return _csrf_error()
+    title = title.strip()
+    if not title or len(title) > 255:
+        return JSONResponse(
+            {"ok": False, "error": "Titre requis (255 caractères max)"},
+            status_code=400,
+        )
+    due_iso, ok = _milestone_due_iso(due_on)
+    if not ok:
+        return JSONResponse(
+            {"ok": False, "error": "Échéance invalide (format AAAA-MM-JJ)"},
+            status_code=400,
+        )
+    client, _cid, _ws = await _resolve_board_scope(request, owner, repo)
+    try:
+        milestone = await client.update_milestone(
+            owner, repo, milestone_id, title=title,
+            due_on=due_iso, clear_due=due_iso is None,
+        )
+    except httpx.HTTPStatusError as exc:
+        return JSONResponse(
+            {"ok": False, "error": f"La forge répond {exc.response.status_code}"},
+            status_code=502,
+        )
+    return {"ok": True, "milestone": {
+        "id": milestone.get("id"),
+        "title": milestone.get("title"),
+        "due_on": milestone.get("due_on"),
+    }}
+
+
+@app.post("/api/repos/{owner}/{repo}/milestones/{milestone_id}/close")
+async def api_milestone_close(
+    request: Request,
+    owner: str,
+    repo: str,
+    milestone_id: int,
+    csrf_token: str = Form(""),
+):
+    """Close a milestone on the forge (issue #26)."""
+    if not _csrf_valid(request, csrf_token):
+        return _csrf_error()
+    client, _cid, _ws = await _resolve_board_scope(request, owner, repo)
+    try:
+        await client.update_milestone(
+            owner, repo, milestone_id, state="closed"
+        )
+    except httpx.HTTPStatusError as exc:
+        return JSONResponse(
+            {"ok": False, "error": f"La forge répond {exc.response.status_code}"},
+            status_code=502,
+        )
+    return {"ok": True}
+
+
 # ── Briefing ───────────────────────────────────────────────
 
 async def _briefing_data() -> dict: