Add database indexes for weather data queries

- Add partial index on temperature column for weather stations
- Add compound index on received_at + temperature for time-based queries
- Add sender + temperature index for station-specific weather queries
- Add individual indexes on humidity, pressure, and wind_speed
- All indexes use WHERE clause to only include non-null values

These indexes will significantly improve performance for:
- Finding weather stations on the map
- Weather history queries
- Weather data filtering

🤖 Generated with [Claude Code](https://claude.ai/code)

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

priv/repo/migrations/20250721190613_add_weather_indexes.exs

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+defmodule Aprsme.Repo.Migrations.AddWeatherIndexes do
+  use Ecto.Migration
+
+  def up do
+    # Index for finding stations with weather data
+    create index(:packets, [:temperature], where: "temperature IS NOT NULL")
+
+    # Compound index for time-based weather queries
+    create index(:packets, [:received_at, :temperature], where: "temperature IS NOT NULL")
+
+    # Additional weather-related indexes for common queries
+    create index(:packets, [:sender, :temperature], where: "temperature IS NOT NULL")
+    create index(:packets, [:humidity], where: "humidity IS NOT NULL")
+    create index(:packets, [:pressure], where: "pressure IS NOT NULL")
+    create index(:packets, [:wind_speed], where: "wind_speed IS NOT NULL")
+  end
+
+  def down do
+    drop index(:packets, [:temperature])
+    drop index(:packets, [:received_at, :temperature])
+    drop index(:packets, [:sender, :temperature])
+    drop index(:packets, [:humidity])
+    drop index(:packets, [:pressure])
+    drop index(:packets, [:wind_speed])
+  end
+end