MongoDB Atlas: Go driver connection with context timeouts

# Go driver + Atlas

```go
package main

import (
    "context"
    "os"
    "time"

    "go.mongodb.org/mongo-driver/mongo"
    "go.mongodb.org/mongo-driver/mongo/options"
)

func connectDB() (*mongo.Client, error) {
    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()

    client, err := mongo.Connect(ctx,
        options.Client().
            ApplyURI(os.Getenv("MONGODB_URI")).
            SetMaxPoolSize(20).
            SetServerSelectionTimeout(5*time.Second),
    )
    if err != nil {
        return nil, err
    }
    if err := client.Ping(ctx, nil); err != nil {
        return nil, err
    }
    return client, nil
}
```

## Rules

- Always pass a context with a timeout to `Connect` and `Ping`. Without it a bad network path hangs forever.
- `ApplyURI` parses the Atlas SRV string including credentials and options. Do not hand-assemble hosts.
- Share the returned `*mongo.Client` across goroutines; it is safe for concurrent use and owns the pool.
- On shutdown call `client.Disconnect(ctx)` so pooled connections close cleanly instead of lingering until the server times them out.
- `SetMaxPoolSize` caps total connections. Size it from expected concurrent operations, not from wishful thinking.

## Verify

`Ping` succeeding from the deploy network proves URI, credentials, and IP access list in one shot.