Add timezone field to Location model and GraphQL API

[Copilot]

This PR adds a timezone field to the sensor location data table to enable proper timezone context for sensor data across different geographical locations.

Changes Made

Database Layer

• Added timezone field as Column(String(50)) to the Location model in app/database/models.py
• Created Alembic migration e3f8f2a9b5c1_add_timezone_to_locations.py to add the timezone column to the api_locations table
• Field is optional (nullable) to maintain backward compatibility with existing data

GraphQL API Layer

• Added timezone: Optional[str] field to the Location GraphQL type
• Updated CreateLocationInput and UpdateLocationInput to accept timezone parameter
• Modified Location.from_model() method to include timezone field mapping
• Updated create_location and update_location resolvers to handle timezone field

Usage

The timezone field supports standard IANA timezone identifiers and can be used in GraphQL operations:

# Query locations with timezone information
query {
  locations {
    id
    name
    city
    country
    timezone
  }
}

# Create a location with timezone
mutation {
  createLocation(input: {
    name: "Tokyo Office"
    city: "Tokyo"
    country: "Japan"
    timezone: "Asia/Tokyo"
  }) {
    id
    name
    timezone
  }
}

Supported Timezone Formats

• UTC/GMT: "UTC", "GMT"
• Americas: "America/New_York", "America/Los_Angeles"
• Europe: "Europe/London", "Europe/Berlin"
• Asia: "Asia/Tokyo", "Asia/Shanghai"
• Australia: "Australia/Sydney"

This enhancement enables accurate time-series data interpretation by providing proper timezone context for sensor locations, which is essential for multi-timezone deployments.

Fixes #17.

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