feat: agent memory management — CRUD API + Android management screen

Backend:
- New /api/v1/agents/{id}/memory endpoints: CRUD for global_knowledge,
  knowledge_entities, learning_patterns, vector_memories + import/export
- Fix scope_id column overflow: 3 model columns expanded to hold compound
  keys (user_id:agent_id format, 73 chars vs old VARCHAR(36))
- Config: allow unknown env vars (extra="ignore") for optional overrides

Android:
- MemoryManageScreen: 4-tab UI (全局知识/知识实体/学习模式/对话记忆)
  with search, delete, and FAB to add new entries
- Import/export via ShareSheet and file picker
- AgentListScreen: long-press dropdown menu → 记忆管理 entry point
- NavGraph: memory_manage/{agentId}/{agentName} route with URL encoding

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -151,6 +151,7 @@ class Settings(BaseSettings):
     class Config:
         env_file = str(_ENV_PATH)
         case_sensitive = True
+        extra = "ignore"
 
 
 settings = Settings()