Rewrite Pastel from Python to Go with watchlist feature

Full rewrite of the gaming deals Matrix bot in Go for better performance
and a single static binary. Includes all original functionality (CheapShark,
ITAD, Epic free games, multi-currency pricing, dedup, preflight checks,
presence heartbeat) plus a new watchlist feature where users can DM the bot
to watch for specific game deals with 180-day auto-expiry.

New: systemd service file, DB migration script for Python-to-Go transition.

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

internal/config/config.go

@@ -0,0 +1,115 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/joho/godotenv"
+)
+
+type Config struct {
+	MatrixHomeserverURL  string
+	MatrixBotUserID      string
+	MatrixBotAccessToken string
+	MatrixDealsRoomID    string
+	ITADAPIKey           string
+	DealSources          []string
+	MinDealRating        float64
+	MinDiscountPercent   int
+	MaxPriceUSD          float64
+	SendIntroMessage     bool
+	DatabasePath         string
+}
+
+func Load() (*Config, error) {
+	// Load .env file if present (ignore error if missing)
+	_ = godotenv.Load()
+
+	c := &Config{}
+	var err error
+
+	c.MatrixHomeserverURL, err = require("MATRIX_HOMESERVER_URL")
+	if err != nil {
+		return nil, err
+	}
+	c.MatrixBotUserID, err = require("MATRIX_BOT_USER_ID")
+	if err != nil {
+		return nil, err
+	}
+	c.MatrixBotAccessToken, err = require("MATRIX_BOT_ACCESS_TOKEN")
+	if err != nil {
+		return nil, err
+	}
+	c.MatrixDealsRoomID, err = require("MATRIX_DEALS_ROOM_ID")
+	if err != nil {
+		return nil, err
+	}
+
+	c.ITADAPIKey = os.Getenv("ITAD_API_KEY")
+
+	rawSources := os.Getenv("DEAL_SOURCES")
+	if rawSources == "" {
+		rawSources = "cheapshark"
+	}
+	for _, s := range strings.Split(rawSources, ",") {
+		s = strings.TrimSpace(strings.ToLower(s))
+		if s != "" {
+			c.DealSources = append(c.DealSources, s)
+		}
+	}
+
+	c.MinDealRating = envFloat("MIN_DEAL_RATING", 8.0)
+	c.MinDiscountPercent = envInt("MIN_DISCOUNT_PERCENT", 50)
+	c.MaxPriceUSD = envFloat("MAX_PRICE_USD", 20)
+	c.DatabasePath = envStr("DATABASE_PATH", "deals.db")
+
+	intro := strings.ToLower(os.Getenv("SEND_INTRO_MESSAGE"))
+	c.SendIntroMessage = intro == "true" || intro == "1" || intro == "yes"
+
+	return c, nil
+}
+
+// HasSource checks if a deal source is enabled.
+func (c *Config) HasSource(name string) bool {
+	for _, s := range c.DealSources {
+		if s == name {
+			return true
+		}
+	}
+	return false
+}
+
+func require(key string) (string, error) {
+	v := os.Getenv(key)
+	if v == "" {
+		return "", fmt.Errorf("required environment variable %s is not set", key)
+	}
+	return v, nil
+}
+
+func envStr(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func envFloat(key string, def float64) float64 {
+	if v := os.Getenv(key); v != "" {
+		if f, err := strconv.ParseFloat(v, 64); err == nil {
+			return f
+		}
+	}
+	return def
+}
+
+func envInt(key string, def int) int {
+	if v := os.Getenv(key); v != "" {
+		if i, err := strconv.Atoi(v); err == nil {
+			return i
+		}
+	}
+	return def
+}