Make the whole configuration settable from the environment

[jmeekhof]

The bug

dns.provider has no environment binding. cmd.go binds TAILSCALE_API_KEY, TAILSCALE_TAILNET, DNS_ZONE_ID, DNS_DOMAIN, CLOUDFLARE_API_TOKEN, AWS_PROFILE, AWS_REGION and the PIHOLE_* pair — but never dns.provider.

viper.AutomaticEnv() doesn't cover the gap, because no key replacer is configured: viper looks for a variable literally named DNS.PROVIDER, so DNS_PROVIDER never maps to the nested key.

Since Config.Validate() hard-requires dns.provider, a config file is mandatory even when every other setting comes from the environment. For a container deployment that otherwise needs no file, that means bind-mounting a file containing a single line.

The README's own "Environment Variables" section shows the symptom — it has to pass the provider as a flag alongside the exported variables:

export TAILSCALE_API_KEY="tskey-api-xxxxx"
...
./dnsscale --dns-provider cloudflare   # <- the one setting that can't come from the env

The same gap silently affects app.workers, app.poll_interval, app.required_tags, logging.level and logging.format, none of which were bound either.

What changed

• Register a strings.NewReplacer(".", "_") key replacer, so nested keys are addressable as DNS_PROVIDER, LOGGING_LEVEL and so on.
• Bind every configuration key explicitly. AutomaticEnv only resolves keys viper already knows about, which makes the set of settings that actually work from the environment hard to predict; an explicit table means the mapping is the same whether or not a config file was loaded, and it's greppable.
• AWS_PROFILE and AWS_REGION are kept rather than moving to the DNS_ROUTE53_* names the replacer would otherwise imply — those are the conventional names and the AWS SDK reads them anyway.
• Document the full key-to-variable mapping in the README, and drop the --dns-provider flag from the env-only examples.

No behaviour changes for existing config files or flags; precedence is unchanged (an explicitly-set flag still wins over the environment).

Tests

Adds cmd_test.go, which unmarshals and validates a configuration built purely from environment variables with no file present. Against the current binding set that test fails with exactly the reported symptom:

cmd_test.go:38: dns.provider = "", want route53
cmd_test.go:70: Validate() on env-only config: dns.provider is required

go build ./..., go vet ./... and go test ./... all pass.