Begin Go rewrite: remove Python, scaffold module and config

Pivot from FastAPI web portal to a Matrix command bot (modeled on Pete).
Users will issue !movie / !tv / !music commands in allowlisted rooms; the
bot performs a top-hit search against Radarr/Sonarr/Lidarr and adds it.

This commit is session 1 of a multi-session rewrite (see SESSION_PLAN.md):
  - Delete app/, requirements.txt, old Dockerfile, .env.example
  - Add go.mod (mautrix-go + yaml.v3)
  - Add internal/config: YAML loader with ${ENV} expansion, validates
    matrix creds, allowed_rooms, and per-service *arr config
  - Reset .gitignore / .dockerignore for the Go layout

internal/config/config.go

@@ -0,0 +1,127 @@
+package config
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"regexp"
+
+	"gopkg.in/yaml.v3"
+)
+
+var envBracketRe = regexp.MustCompile(`\$\{([^}]+)\}`)
+
+type Config struct {
+	Matrix   MatrixConfig   `yaml:"matrix"`
+	Services ServicesConfig `yaml:"services"`
+}
+
+type MatrixConfig struct {
+	Homeserver  string   `yaml:"homeserver"`
+	UserID      string   `yaml:"user_id"`
+	Password    string   `yaml:"password"`
+	PickleKey   string   `yaml:"pickle_key"`
+	DisplayName string   `yaml:"display_name"`
+	DataDir     string   `yaml:"data_dir"`
+	CommandPrefix string `yaml:"command_prefix"`
+	// AllowedRooms is the set of room IDs the bot listens for commands in.
+	// Messages in any other room are ignored.
+	AllowedRooms []string `yaml:"allowed_rooms"`
+}
+
+type ServicesConfig struct {
+	Radarr *ArrConfig `yaml:"radarr"`
+	Sonarr *ArrConfig `yaml:"sonarr"`
+	Lidarr *ArrConfig `yaml:"lidarr"`
+}
+
+type ArrConfig struct {
+	URL              string `yaml:"url"`
+	APIKey           string `yaml:"api_key"`
+	QualityProfileID int    `yaml:"quality_profile_id"`
+	RootFolder       string `yaml:"root_folder"`
+}
+
+func Load(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read config: %w", err)
+	}
+
+	expanded := envBracketRe.ReplaceAllStringFunc(string(data), func(match string) string {
+		varName := match[2 : len(match)-1]
+		val := os.Getenv(varName)
+		if val == "" {
+			slog.Warn("config: env var referenced but not set", "var", varName)
+		}
+		return val
+	})
+
+	var cfg Config
+	if err := yaml.Unmarshal([]byte(expanded), &cfg); err != nil {
+		return nil, fmt.Errorf("parse config: %w", err)
+	}
+
+	cfg.applyDefaults()
+
+	if err := cfg.validate(); err != nil {
+		return nil, fmt.Errorf("validate config: %w", err)
+	}
+
+	return &cfg, nil
+}
+
+func (c *Config) validate() error {
+	if c.Matrix.Homeserver == "" {
+		return fmt.Errorf("matrix.homeserver is required")
+	}
+	if c.Matrix.UserID == "" {
+		return fmt.Errorf("matrix.user_id is required")
+	}
+	if c.Matrix.Password == "" {
+		return fmt.Errorf("matrix.password is required")
+	}
+	if len(c.Matrix.AllowedRooms) == 0 {
+		return fmt.Errorf("matrix.allowed_rooms must have at least one entry")
+	}
+	if c.Services.Radarr == nil && c.Services.Sonarr == nil && c.Services.Lidarr == nil {
+		return fmt.Errorf("at least one of services.radarr/sonarr/lidarr must be configured")
+	}
+	for name, svc := range map[string]*ArrConfig{
+		"radarr": c.Services.Radarr,
+		"sonarr": c.Services.Sonarr,
+		"lidarr": c.Services.Lidarr,
+	} {
+		if svc == nil {
+			continue
+		}
+		if svc.URL == "" {
+			return fmt.Errorf("services.%s.url is required when service is set", name)
+		}
+		if svc.APIKey == "" {
+			return fmt.Errorf("services.%s.api_key is required when service is set", name)
+		}
+		if svc.RootFolder == "" {
+			return fmt.Errorf("services.%s.root_folder is required when service is set", name)
+		}
+		if svc.QualityProfileID == 0 {
+			return fmt.Errorf("services.%s.quality_profile_id is required when service is set", name)
+		}
+	}
+	return nil
+}
+
+func (c *Config) applyDefaults() {
+	if c.Matrix.DataDir == "" {
+		c.Matrix.DataDir = "./data"
+	}
+	if c.Matrix.DisplayName == "" {
+		c.Matrix.DisplayName = "Bellhop"
+	}
+	if c.Matrix.PickleKey == "" {
+		c.Matrix.PickleKey = "bellhop_pickle_key"
+	}
+	if c.Matrix.CommandPrefix == "" {
+		c.Matrix.CommandPrefix = "!"
+	}
+}