feat: v0.9.0 — Backend Notion-Style (propriétés custom, AI keywords, sync)

DB:
- project_properties: propriétés custom par projet (select, multi_select, date, person, text)
- property_values: valeurs par issue (UNIQUE property+issue)
- ai_keywords: mots-clés extraits automatiquement (keyword, color, usage_count)

Backend:
- /board/api/properties/{o}/{r} — CRUD propriétés custom
- /board/api/ai-keywords/{o}/{r} — GET keywords list
- /board/api/ai-keywords/{o}/{r}/extract — POST ré-extraction
- /board/api/sync/{o}/{r} — POST sync bidirectionnelle complète
- _map_issue_to_card enrichi: custom_props + keywords par issue
- _extract_ai_keywords: extraction depuis labels + body texte

Documentation:
- ROADMAP v0.9.0 ✅ (5/5 checkboxes)

app/routers/api.py

@@ -421,7 +421,7 @@ async def get_issue_detail(owner: str, repo: str, issue_id: int, format: str = Q
 
     # HTML format for modal
     if format == "html":
-        card_data = _map_issue_to_card(issue)
+        card_data = _map_issue_to_card(issue, owner, repo)
         ctx = {
             "issue": {
                 "id": issue_id,