config: use /var/lib/dnswatcher as default data directory (#89)

Closes [issue #88](#88).

Changes the default `DNSWATCHER_DATA_DIR` from the relative path `./data` to the absolute path `/var/lib/dnswatcher`, following the [Filesystem Hierarchy Standard](https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch05s08.html) convention for variable application state data.

## Changes

- **`internal/config/config.go`**: Changed the Viper default for `DATA_DIR` from `"./data"` to `"/var/lib/"+name`, where `name` is the application name ("dnswatcher"). This makes the default derived from the app name rather than hardcoded.
- **`internal/config/config_test.go`**: Updated `TestNew_DefaultValues` and `TestStatePath` to expect the new absolute default.
- **`README.md`**: Updated the environment variable table and `.env` example to show `/var/lib/dnswatcher` as the default.

The Dockerfile already set `ENV DNSWATCHER_DATA_DIR=/var/lib/dnswatcher` explicitly, so Docker deployments are unaffected. This change makes the code default consistent with the Docker configuration.

`docker build .` passes all checks (fmt, lint, tests, build).

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #89
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/config/config_test.go

@@ -44,7 +44,7 @@ func TestNew_DefaultValues(t *testing.T) {
 
 	assert.Equal(t, 8080, cfg.Port)
 	assert.False(t, cfg.Debug)
-	assert.Equal(t, "./data", cfg.DataDir)
+	assert.Equal(t, "/var/lib/dnswatcher", cfg.DataDir)
 	assert.Equal(t, time.Hour, cfg.DNSInterval)
 	assert.Equal(t, 12*time.Hour, cfg.TLSInterval)
 	assert.Equal(t, 7, cfg.TLSExpiryWarning)
@@ -245,7 +245,7 @@ func TestStatePath(t *testing.T) {
 		dataDir string
 		want    string
 	}{
-		{"default", "./data", "./data/state.json"},
+		{"default", "/var/lib/dnswatcher", "/var/lib/dnswatcher/state.json"},
 		{"absolute", "/var/lib/dw", "/var/lib/dw/state.json"},
 		{"nested", "/opt/app/data", "/opt/app/data/state.json"},
 		{"empty", "", "/state.json"},