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

go.mod

@@ -14,35 +14,22 @@ require (
 	github.com/joho/godotenv v1.5.1
 	github.com/prometheus/client_golang v1.18.0
 	github.com/slok/go-http-metrics v0.11.0
-	github.com/spf13/afero v1.14.0
 	github.com/stretchr/testify v1.8.4
 	go.uber.org/fx v1.20.1
 	golang.org/x/crypto v0.38.0
 	gorm.io/driver/sqlite v1.5.4
 	gorm.io/gorm v1.25.5
 	modernc.org/sqlite v1.28.0
-	sneak.berlin/go/webhooker/pkg/config v0.0.0-00010101000000-000000000000
 )
 
 require (
-	cloud.google.com/go/compute v1.23.3 // indirect
-	cloud.google.com/go/compute/metadata v0.2.3 // indirect
-	cloud.google.com/go/iam v1.1.5 // indirect
-	cloud.google.com/go/secretmanager v1.11.4 // indirect
-	github.com/aws/aws-sdk-go v1.50.0 // indirect
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.2.0 // indirect
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
-	github.com/golang/groupcache v0.0.0-20210331224755-41bb18bfe9da // indirect
-	github.com/golang/protobuf v1.5.3 // indirect
-	github.com/google/s2a-go v0.1.7 // indirect
-	github.com/googleapis/enterprise-certificate-proxy v0.3.2 // indirect
-	github.com/googleapis/gax-go/v2 v2.12.0 // indirect
 	github.com/gorilla/securecookie v1.1.2 // indirect
 	github.com/jinzhu/inflection v1.0.0 // indirect
 	github.com/jinzhu/now v1.1.5 // indirect
-	github.com/jmespath/go-jmespath v0.4.0 // indirect
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51 // indirect
 	github.com/kr/text v0.2.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
@@ -53,25 +40,15 @@ require (
 	github.com/prometheus/common v0.45.0 // indirect
 	github.com/prometheus/procfs v0.12.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	go.opencensus.io v0.24.0 // indirect
 	go.uber.org/atomic v1.9.0 // indirect
 	go.uber.org/dig v1.17.0 // indirect
 	go.uber.org/multierr v1.9.0 // indirect
 	go.uber.org/zap v1.23.0 // indirect
 	golang.org/x/mod v0.17.0 // indirect
-	golang.org/x/net v0.25.0 // indirect
-	golang.org/x/oauth2 v0.15.0 // indirect
 	golang.org/x/sync v0.14.0 // indirect
 	golang.org/x/sys v0.33.0 // indirect
 	golang.org/x/text v0.25.0 // indirect
-	golang.org/x/time v0.5.0 // indirect
 	golang.org/x/tools v0.21.1-0.20240508182429-e35e4ccd0d2d // indirect
-	google.golang.org/api v0.153.0 // indirect
-	google.golang.org/appengine v1.6.7 // indirect
-	google.golang.org/genproto v0.0.0-20231106174013-bbf56f31fb17 // indirect
-	google.golang.org/genproto/googleapis/api v0.0.0-20231106174013-bbf56f31fb17 // indirect
-	google.golang.org/genproto/googleapis/rpc v0.0.0-20231120223509-83a465c0220f // indirect
-	google.golang.org/grpc v1.59.0 // indirect
 	google.golang.org/protobuf v1.31.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	lukechampine.com/uint128 v1.2.0 // indirect
@@ -84,5 +61,3 @@ require (
 	modernc.org/strutil v1.1.3 // indirect
 	modernc.org/token v1.0.1 // indirect
 )
-
-replace sneak.berlin/go/webhooker/pkg/config => ./pkg/config