feat: add configuration management with YAML parsing and validation

Add internal/config package with Config struct (Server, Navidrome,
MusicBrainz, Telegram, Scanner sections), LoadConfig function for
YAML parsing, config validation (required fields, port range,
threshold range), and defaults. Include config.yaml.example matching
spec. Update main.go to use real config package instead of
placeholders. Add comprehensive tests covering valid configs,
defaults, missing files, malformed YAML, and validation boundaries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

go.mod

@@ -1,3 +1,5 @@
 module naviwatcher
 
 go 1.25.1
+
+require gopkg.in/yaml.v3 v3.0.1 // indirect