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

README.md

@@ -50,17 +50,12 @@ make hooks      # Install git pre-commit hook that runs make check
 
 ### Configuration
 
-webhooker uses a YAML configuration file with environment-specific
-overrides, loaded via the `pkg/config` library (Viper-based). The
-environment is selected by setting `WEBHOOKER_ENVIRONMENT` to `dev` or
-`prod` (default: `dev`).
+All configuration is via environment variables. For local development,
+you can place variables in a `.env` file in the project root (loaded
+automatically via `godotenv/autoload`).
 
-Configuration is resolved in this order (highest priority first):
-
-1. Environment variables
-2. `.env` file (loaded via `godotenv/autoload`)
-3. Config file values for the active environment
-4. Config file defaults
+The environment is selected by setting `WEBHOOKER_ENVIRONMENT` to `dev`
+or `prod` (default: `dev`).
 
 | Variable                | Description                         | Default  |
 | ----------------------- | ----------------------------------- | -------- |
@@ -692,7 +687,7 @@ webhooker/
 │   └── main.go               # Entry point: sets globals, wires fx
 ├── internal/
 │   ├── config/
-│   │   └── config.go          # Configuration loading via pkg/config
+│   │   └── config.go          # Configuration loading from environment variables
 │   ├── database/
 │   │   ├── base_model.go      # BaseModel with UUID primary keys
 │   │   ├── database.go        # GORM connection, migrations, admin seed
@@ -733,14 +728,11 @@ webhooker/
 │   │   └── routes.go          # All route definitions
 │   └── session/
 │       └── session.go         # Cookie-based session management
-├── pkg/config/                # Reusable multi-environment config library
 ├── static/
 │   ├── static.go              # //go:embed directive
 │   ├── css/style.css          # Custom stylesheet (system font stack, card effects, layout)
 │   └── js/app.js              # Client-side JavaScript (minimal bootstrap)
 ├── templates/                 # Go HTML templates (base, index, login, etc.)
-├── configs/
-│   └── config.yaml.example    # Example configuration file
 ├── Dockerfile                 # Multi-stage: build + check, then Alpine runtime
 ├── Makefile                   # fmt, lint, test, check, build, docker targets
 ├── go.mod / go.sum
@@ -753,7 +745,7 @@ Components are wired via Uber fx in this order:
 
 1. `globals.New` — Build-time variables (appname, version, arch)
 2. `logger.New` — Structured logging (slog with TTY detection)
-3. `config.New` — Configuration loading (pkg/config + environment)
+3. `config.New` — Configuration loading (environment variables)
 4. `database.New` — Main SQLite connection, config migrations, admin
    user seed
 5. `database.NewWebhookDBManager` — Per-webhook event database