fix: Remove non-immutable NOW() function from index predicate

PostgreSQL requires functions in index predicates to be immutable.
The NOW() function is not immutable, so we simplify the index to
just order by received_at DESC without the WHERE clause.

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

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

priv/repo/migrations/20250730185642_optimize_packet_queries.exs

@@ -27,11 +27,11 @@ defmodule Aprsme.Repo.Migrations.OptimizePacketQueries do
     WHERE device_identifier IS NOT NULL
     """
 
-    # Partial index for recent packets (last 7 days) - very useful for most queries
+    # Index for recent packets - using simple timestamp ordering
+    # Note: We can't use NOW() in the WHERE clause as it's not immutable
     execute """
     CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_packets_recent_only 
     ON packets(received_at DESC)
-    WHERE received_at > NOW() - INTERVAL '7 days'
     """
 
     # Index for weather packet lookups by callsign