feat(11-01): create sites table migration, model, and schemas

- Add migration 030 with sites table, RLS policy, and device site_id FK
- Add Site SQLAlchemy model with tenant isolation
- Add site_id nullable FK and relationship to Device model
- Add sites relationship to Tenant model
- Register Site in models __init__.py
- Add SiteCreate, SiteUpdate, SiteResponse, SiteListResponse schemas

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/site.py

@@ -0,0 +1,74 @@
+"""Pydantic schemas for Site endpoints."""
+
+import uuid
+from datetime import datetime
+from typing import Optional
+
+from pydantic import BaseModel, field_validator
+
+
+class SiteCreate(BaseModel):
+    """Schema for creating a new site."""
+
+    name: str
+    latitude: Optional[float] = None
+    longitude: Optional[float] = None
+    address: Optional[str] = None
+    elevation: Optional[float] = None
+    notes: Optional[str] = None
+
+    @field_validator("name")
+    @classmethod
+    def validate_name(cls, v: str) -> str:
+        v = v.strip()
+        if len(v) < 1 or len(v) > 255:
+            raise ValueError("Site name must be 1-255 characters")
+        return v
+
+
+class SiteUpdate(BaseModel):
+    """Schema for updating an existing site. All fields optional."""
+
+    name: Optional[str] = None
+    latitude: Optional[float] = None
+    longitude: Optional[float] = None
+    address: Optional[str] = None
+    elevation: Optional[float] = None
+    notes: Optional[str] = None
+
+    @field_validator("name")
+    @classmethod
+    def validate_name(cls, v: Optional[str]) -> Optional[str]:
+        if v is None:
+            return v
+        v = v.strip()
+        if len(v) < 1 or len(v) > 255:
+            raise ValueError("Site name must be 1-255 characters")
+        return v
+
+
+class SiteResponse(BaseModel):
+    """Site response schema with health rollup stats."""
+
+    id: uuid.UUID
+    name: str
+    latitude: Optional[float] = None
+    longitude: Optional[float] = None
+    address: Optional[str] = None
+    elevation: Optional[float] = None
+    notes: Optional[str] = None
+    device_count: int = 0
+    online_count: int = 0
+    online_percent: float = 0.0
+    alert_count: int = 0
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class SiteListResponse(BaseModel):
+    """List of sites with unassigned device count."""
+
+    sites: list[SiteResponse]
+    unassigned_count: int