feat(v5.9.0): AI Writing Assist - slash /ai, autocompletion, AI properties

- Service app/services/ai_writing.py: 6 actions sans outils (write, summarize,
  translate, continue, autocomplete, properties) + replis deterministes offline
- Endpoints POST /api/agent/writing et /api/agent/writing/properties
- Editeur: groupe slash AI (Write/Summarize/Translate/Continue) via E.aiSlash
- Autocompletion inline AIAC (suggestion ~900ms, Tab accepte, Escape rejette)
- Database: bouton AI fill (suggestions de proprietes Status/Priority/Resume)
- Tests tests/test_ai_writing.py (+29) ; version 5.9.0 (VERSION, main.py)
- CHANGELOG + ROADMAP v5.9.0 completes

app/routers/agent.py

@@ -367,6 +367,82 @@ async def agent_generate(request: Request):
     }
 
 
+# ── AI Writing Assist (v5.9.0) ──
+
+
+@router.post("/writing")
+async def agent_writing(request: Request):
+    """Headless writing actions for the editor (v5.9.0).
+
+    `action` is one of: write, summarize, translate, continue, autocomplete.
+    Returns plain Markdown (`text`) plus `model`/`offline` metadata. The
+    `properties` action is served by `/writing/properties`.
+    """
+    from app.services.ai_writing import AIWritingService, WRITING_ACTIONS
+
+    user_id = await _current_user_id(request)
+    body = await request.json() if request.headers.get("content-type") else {}
+    action = (body.get("action") or "").strip().lower()
+    if not action:
+        raise HTTPException(status_code=400, detail="action est requis")
+    if action == "properties":
+        raise HTTPException(status_code=400, detail="Utilisez /writing/properties pour l'action properties")
+    if action not in WRITING_ACTIONS:
+        raise HTTPException(status_code=400, detail=f"Action inconnue: {action}")
+
+    prompt = (body.get("prompt") or "").strip()
+    if action == "write" and not prompt and not (body.get("title") or "").strip():
+        raise HTTPException(status_code=400, detail="prompt est requis pour l'action write")
+
+    service = AIWritingService(
+        user_id=user_id,
+        provider=(body.get("provider") or "").strip() or None,
+        model=(body.get("model") or "").strip() or None,
+    )
+    result = await service.run(
+        action,
+        prompt=prompt,
+        context=(body.get("context") or ""),
+        target_language=(body.get("target_language") or "English"),
+        prefix=(body.get("prefix") or ""),
+        title=(body.get("title") or ""),
+    )
+    return result
+
+
+@router.post("/writing/properties")
+async def agent_writing_properties(request: Request):
+    """Suggest values for a collection page's properties (v5.9.0).
+
+    Body: `{title, content?, properties: [{name, type}], provider?, model?}`.
+    Returns `{ok, suggestions: {property_name: value}, model, offline}`.
+    """
+    from app.services.ai_writing import AIWritingService
+
+    user_id = await _current_user_id(request)
+    body = await request.json() if request.headers.get("content-type") else {}
+    properties = body.get("properties") or []
+    if not isinstance(properties, list) or not properties:
+        raise HTTPException(status_code=400, detail="properties est requis (liste non vide)")
+
+    service = AIWritingService(
+        user_id=user_id,
+        provider=(body.get("provider") or "").strip() or None,
+        model=(body.get("model") or "").strip() or None,
+    )
+    suggestions = await service.suggest_properties(
+        context=(body.get("content") or ""),
+        title=(body.get("title") or ""),
+        properties=properties,
+    )
+    return {
+        "ok": True,
+        "suggestions": suggestions,
+        "model": service.model or "",
+        "offline": service._offline_hint(),
+    }
+
+
 # ── Audit & rollback ──