feat(config): register all defaults in code, shrink example config

Only the database password is truly mandatory to get started, but
config.example.json was 267 lines with 90+ options. Newcomers faced a
wall of settings with no indication of what matters.

Add registerDefaults() with all sane defaults via Viper so a minimal
config (just DB credentials) produces a fully working server. Gameplay
multipliers default to 1.0 instead of Go's zero value 0.0, which
previously zeroed out all quest rewards for minimal configs. Uses
dot-notation defaults for GameplayOptions/DebugOptions so users can
override individual fields without losing other defaults.

- Shrink config.example.json from 267 to 10 lines
- Rename full original to config.reference.json as documentation
- Simplify wizard buildDefaultConfig() from ~220 to ~12 lines
- Fix latent bug: SaveDumps default used wrong key DevModeOptions
- Add tests: minimal config, backward compat, single-field override
- Update release workflow, README, CONTRIBUTING, docker/README

CONTRIBUTING.md

@@ -21,7 +21,7 @@ Thank you for your interest in contributing to Erupe! This guide will help you g
    ```
 
 3. Set up the database following the [Installation guide](README.md#installation)
-4. Copy `config.example.json` to `config.json` and configure it
+4. Copy `config.example.json` to `config.json` and set your database password (see `config.reference.json` for all available options)
 5. Install dependencies:
 
    ```bash