setup: add Viper config loading and internal package structure

[Lewiskunta]

Implements - environment variable loading via Viper.

Changes:

• Added github.com/spf13/viper and github.com/spf13/cobra
• Created internal/config/config.go with full Config struct
• Config reads from .env file, environment variables, and defaults
• Validation for ENCRYPTION_KEY length, PORT range, required fields
• Helper methods: IsProduction(), IsDevelopment(), BreachInterval()
• Unit tests covering defaults, env var overrides, validation errors
• Updated cmd/server/main.go to load config and log startup info
• Structured JSON logging via log/slog (Go standard library)

After this PR, the server reads all configuration from environment variables. No hardcoded values anywhere.