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/db.py

@@ -99,6 +99,37 @@ def init_db():
                 position INTEGER NOT NULL DEFAULT 0,
                 created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
             );
+
+            CREATE TABLE IF NOT EXISTS project_properties (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                project_owner TEXT NOT NULL,
+                project_name TEXT NOT NULL,
+                name TEXT NOT NULL,
+                prop_type TEXT NOT NULL DEFAULT 'select',  -- select, multi_select, date, person, text
+                options_json TEXT DEFAULT '[]',             -- for select/multi_select
+                position INTEGER NOT NULL DEFAULT 0,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+                UNIQUE(project_owner, project_name, name)
+            );
+
+            CREATE TABLE IF NOT EXISTS property_values (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                property_id INTEGER NOT NULL REFERENCES project_properties(id) ON DELETE CASCADE,
+                gitea_issue_id INTEGER NOT NULL,
+                value TEXT NOT NULL DEFAULT '',
+                UNIQUE(property_id, gitea_issue_id)
+            );
+
+            CREATE TABLE IF NOT EXISTS ai_keywords (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                project_owner TEXT NOT NULL,
+                project_name TEXT NOT NULL,
+                keyword TEXT NOT NULL,
+                color TEXT NOT NULL DEFAULT '#787774',
+                usage_count INTEGER NOT NULL DEFAULT 1,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+                UNIQUE(project_owner, project_name, keyword)
+            );
         """)
         conn.commit()