chore: remove redundant migrations directory and migrate binary

All content from 003_add_device_codes.sql and 004_add_trigram_indexes.sql
was already absorbed into db/schema.sql (the single source of truth).

The migrations had additional problems:
- Broken numbering (started at 003, missing 001/002)
- 004 used CREATE INDEX CONCURRENTLY which fails inside sqlx transactions
- No production flow ever invoked the migrate binary

Removed: db/migrations/, src/bin/migrate.rs, migrations Cargo feature,
[[bin]] migrate target, and doc/database-migrations.md.

src/bin/migrate.rs

@@ -1,49 +0,0 @@
-use sqlx::postgres::PgPoolOptions;
-use std::env;
-use std::path::Path;
-use std::time::Duration;
-
-#[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error>> {
-    // Configure logging
-    tracing_subscriber::fmt::init();
-
-    // Load environment variables (.env.local first, then .env)
-    if let Ok(path) = env::var("DOTENV_PATH") {
-        dotenvy::from_path(Path::new(&path)).ok();
-    } else {
-        dotenvy::from_filename(".env.local").ok();
-        dotenvy::dotenv().ok();
-    }
-
-    // Get DATABASE_URL from environment variables
-    let database_url = env::var("DATABASE_URL").expect("DATABASE_URL must be configured");
-
-    println!("Connecting to the database...");
-
-    // Create connection pool
-    let pool = PgPoolOptions::new()
-        .max_connections(5)
-        .acquire_timeout(Duration::from_secs(10))
-        .connect(&database_url)
-        .await?;
-
-    // Run migrations
-    println!("Running migrations...");
-
-    // Get the directory from an environment variable or use a default value
-    let migrations_dir = env::var("MIGRATIONS_DIR").unwrap_or_else(|_| "./migrations".to_string());
-    println!("Migrations directory: {}", migrations_dir);
-
-    // Create a migrator
-    let migrator = sqlx::migrate::Migrator::new(Path::new(&migrations_dir))
-        .await
-        .expect("Could not create the migrator");
-
-    // Run all pending migrations
-    migrator.run(&pool).await?;
-
-    println!("Migrations applied successfully");
-
-    Ok(())
-}