Add FastAPI backend with agent system

backend/app/schemas/stats.py

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+from pydantic import BaseModel
+from typing import Optional
+
+
+# ===== System Health =====
+class SystemHealth(BaseModel):
+    uptime_seconds: int
+    cpu_percent: float
+    memory_used_mb: float
+    memory_total_mb: float
+    memory_percent: float
+    disk_used_gb: float
+    disk_total_gb: float
+    disk_percent: float
+    active_users_24h: int
+
+
+# ===== Daily Stats Base =====
+class DailyStatItem(BaseModel):
+    date: str
+    count: int
+
+
+class DailyTokenStatItem(BaseModel):
+    date: str
+    input_tokens: int
+    output_tokens: int
+
+
+# ===== Conversation Stats =====
+class ConversationStats(BaseModel):
+    daily_conversations: list[DailyStatItem]
+    daily_messages: list[DailyStatItem]
+    daily_input_tokens: list[DailyTokenStatItem]
+    daily_output_tokens: list[DailyTokenStatItem]
+    totals: dict
+
+
+# ===== Knowledge Stats =====
+class KnowledgeStats(BaseModel):
+    daily_new_tags: list[DailyStatItem]
+    daily_documents: list[DailyStatItem]
+    daily_knowledge_queries: list[DailyStatItem]
+    daily_tag_relations: list[DailyStatItem]
+    totals: dict
+
+
+# ===== Kanban Stats =====
+class KanbanStats(BaseModel):
+    daily_new_tasks: list[DailyStatItem]
+    daily_completed_tasks: list[DailyStatItem]
+    daily_completion_rate: list[DailyStatItem]
+    current_pending_tasks: int
+    totals: dict
+
+
+# ===== Community Stats =====
+class CommunityStats(BaseModel):
+    daily_posts: list[DailyStatItem]
+    daily_replies: list[DailyStatItem]
+    daily_ai_executions: list[DailyStatItem]
+    daily_agent_calls: list[DailyStatItem]
+    totals: dict
+
+
+# ===== Personal Insights =====
+class HourlyActivity(BaseModel):
+    hour: int
+    count: int
+
+
+class TagUsage(BaseModel):
+    tag_path: str
+    usage_count: int
+
+
+class PersonalInsights(BaseModel):
+    hourly_activity: list[HourlyActivity]
+    top_tags: list[TagUsage]
+    token_trend_percent: float
+    this_month_tokens: int
+    last_month_tokens: int