Add configuration management

[kibirisu]

Summary

The goal is to implement structured configuration management using Viper to handle environment variables and configuration files in a unified way.
This change replaces the early-stage ad-hoc environment variable handling and prepares the codebase for future scalability.

Motivation

As the project grows, simple environment variables are not sufficient to manage configuration. We will need:

• Support for multiple environments (development, staging, production).

• A centralized way to define defaults and load overrides.

• Config files (e.g., YAML or TOML) for local and deployment setups.

Using Viper provides:

• A single API for reading environment variables, flags, and config files.

• Built-in support for defaults and automatic environment binding.

• Future flexibility for reloading or merging configs dynamically.

TODO

• Added dependency on Viper.

• Added runtime selection for Postgres and SQLite backends based on configuration.

• Validate config values and provide sane defaults (e.g. default url, fallback database).

• Add support for configuration files (e.g. config.yaml).

• Integrate config-driven setup into application initialization.

[kibirisu]

Reading app listen port from env vars or envfile added in #10

[kibirisu]

We should consider how the backend will keep track of instance settings across restarts.