refactor: remove file-based configuration, use env vars only

Remove the entire pkg/config package (Viper-based YAML config file
loader) and simplify internal/config to read all settings directly from
environment variables via os.Getenv(). This eliminates the spurious
"Failed to load config" log messages that appeared when no config.yaml
file was present.

- Delete pkg/config/ (YAML loader, resolver, manager, tests)
- Delete configs/config.yaml.example
- Simplify internal/config helper functions to use os.Getenv() with
  defaults instead of falling back to pkgconfig
- Update tests to set env vars directly instead of creating in-memory
  YAML config files via afero
- Remove afero, cloud.google.com/*, aws-sdk-go dependencies from go.mod
- Update README: document env-var-only configuration, remove YAML/Viper
  references
- Keep godotenv/autoload for .env file convenience in local development

closes #27

internal/config/config.go

@@ -10,7 +10,6 @@ import (
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
-	pkgconfig "sneak.berlin/go/webhooker/pkg/config"
 
 	// Populates the environment from a ./.env file automatically for
 	// development configuration. Kept in one place only (here).
@@ -56,38 +55,30 @@ func (c *Config) IsProd() bool {
 	return c.Environment == EnvironmentProd
 }
 
-// envString returns the env var value if set, otherwise falls back to pkgconfig.
-func envString(envKey, configKey string) string {
-	if v := os.Getenv(envKey); v != "" {
-		return v
-	}
-	return pkgconfig.GetString(configKey)
+// envString returns the value of the named environment variable, or
+// an empty string if not set.
+func envString(key string) string {
+	return os.Getenv(key)
 }
 
-// envSecretString returns the env var value if set, otherwise falls back to pkgconfig secrets.
-func envSecretString(envKey, configKey string) string {
-	if v := os.Getenv(envKey); v != "" {
-		return v
-	}
-	return pkgconfig.GetSecretString(configKey)
-}
-
-// envBool returns the env var value parsed as bool, otherwise falls back to pkgconfig.
-func envBool(envKey, configKey string) bool {
-	if v := os.Getenv(envKey); v != "" {
+// envBool returns the value of the named environment variable parsed as a
+// boolean. Returns defaultValue if not set.
+func envBool(key string, defaultValue bool) bool {
+	if v := os.Getenv(key); v != "" {
 		return strings.EqualFold(v, "true") || v == "1"
 	}
-	return pkgconfig.GetBool(configKey)
+	return defaultValue
 }
 
-// envInt returns the env var value parsed as int, otherwise falls back to pkgconfig.
-func envInt(envKey, configKey string, defaultValue ...int) int {
-	if v := os.Getenv(envKey); v != "" {
+// envInt returns the value of the named environment variable parsed as an
+// integer. Returns defaultValue if not set or unparseable.
+func envInt(key string, defaultValue int) int {
+	if v := os.Getenv(key); v != "" {
 		if i, err := strconv.Atoi(v); err == nil {
 			return i
 		}
 	}
-	return pkgconfig.GetInt(configKey, defaultValue...)
+	return defaultValue
 }
 
 // nolint:revive // lc parameter is required by fx even if unused
@@ -106,21 +97,18 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 			EnvironmentDev, EnvironmentProd, environment)
 	}
 
-	// Set the environment in the config package (for fallback resolution)
-	pkgconfig.SetEnvironment(environment)
-
-	// Load configuration values — env vars take precedence over config.yaml
+	// Load configuration values from environment variables
 	s := &Config{
-		DBURL:           envString("DBURL", "dburl"),
-		DataDir:         envString("DATA_DIR", "dataDir"),
-		Debug:           envBool("DEBUG", "debug"),
-		MaintenanceMode: envBool("MAINTENANCE_MODE", "maintenanceMode"),
-		DevelopmentMode: envBool("DEVELOPMENT_MODE", "developmentMode"),
+		DBURL:           envString("DBURL"),
+		DataDir:         envString("DATA_DIR"),
+		Debug:           envBool("DEBUG", false),
+		MaintenanceMode: envBool("MAINTENANCE_MODE", false),
+		DevelopmentMode: envBool("DEVELOPMENT_MODE", false),
 		Environment:     environment,
-		MetricsUsername: envString("METRICS_USERNAME", "metricsUsername"),
-		MetricsPassword: envString("METRICS_PASSWORD", "metricsPassword"),
-		Port:            envInt("PORT", "port", 8080),
-		SentryDSN:       envSecretString("SENTRY_DSN", "sentryDSN"),
+		MetricsUsername: envString("METRICS_USERNAME"),
+		MetricsPassword: envString("METRICS_PASSWORD"),
+		Port:            envInt("PORT", 8080),
+		SentryDSN:       envString("SENTRY_DSN"),
 		log:             log,
 		params:          &params,
 	}