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/database/database.go

@@ -0,0 +1,105 @@
+package database
+
+import (
+	"time"
+
+	"github.com/jmoiron/sqlx"
+	_ "modernc.org/sqlite"
+)
+
+type DB struct {
+	db *sqlx.DB
+}
+
+func Open(path string) (*DB, error) {
+	db, err := sqlx.Open("sqlite", path+"?_pragma=journal_mode(WAL)")
+	if err != nil {
+		return nil, err
+	}
+	if err := db.Ping(); err != nil {
+		return nil, err
+	}
+	d := &DB{db: db}
+	if err := d.migrate(); err != nil {
+		return nil, err
+	}
+	return d, nil
+}
+
+func (d *DB) Close() error {
+	return d.db.Close()
+}
+
+// RawDB exposes the underlying sqlx.DB for packages that need direct access.
+func (d *DB) RawDB() *sqlx.DB {
+	return d.db
+}
+
+func (d *DB) migrate() error {
+	_, err := d.db.Exec(`
+		CREATE TABLE IF NOT EXISTS posted_deals (
+			id TEXT PRIMARY KEY,
+			source TEXT NOT NULL,
+			title TEXT NOT NULL,
+			posted_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+		);
+		CREATE TABLE IF NOT EXISTS config (
+			key TEXT PRIMARY KEY,
+			value TEXT
+		);
+		CREATE TABLE IF NOT EXISTS watchlist (
+			id INTEGER PRIMARY KEY AUTOINCREMENT,
+			user_id TEXT NOT NULL,
+			game_name TEXT NOT NULL,
+			game_name_normalized TEXT NOT NULL,
+			created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+			expires_at TIMESTAMP NOT NULL,
+			expiry_warned INTEGER DEFAULT 0,
+			UNIQUE(user_id, game_name_normalized)
+		);
+		CREATE INDEX IF NOT EXISTS idx_watchlist_normalized ON watchlist(game_name_normalized);
+		CREATE INDEX IF NOT EXISTS idx_watchlist_expires ON watchlist(expires_at);
+	`)
+	return err
+}
+
+// IsPosted checks if a deal has already been posted.
+func (d *DB) IsPosted(id string) (bool, error) {
+	var count int
+	err := d.db.Get(&count, "SELECT COUNT(1) FROM posted_deals WHERE id = ?", id)
+	return count > 0, err
+}
+
+// MarkPosted records a deal as posted.
+func (d *DB) MarkPosted(id, source, title string) error {
+	_, err := d.db.Exec(
+		"INSERT OR IGNORE INTO posted_deals (id, source, title) VALUES (?, ?, ?)",
+		id, source, title,
+	)
+	return err
+}
+
+// IsFirstRunDone checks if the initial population has been completed.
+func (d *DB) IsFirstRunDone() (bool, error) {
+	var val string
+	err := d.db.Get(&val, "SELECT value FROM config WHERE key = 'first_run_done'")
+	if err != nil {
+		return false, nil // not found = not done
+	}
+	return val == "true", nil
+}
+
+// SetFirstRunDone marks the initial population as complete.
+func (d *DB) SetFirstRunDone() error {
+	_, err := d.db.Exec(
+		"INSERT OR REPLACE INTO config (key, value) VALUES ('first_run_done', 'true')",
+	)
+	return err
+}
+
+// PruneOldDeals removes deals older than the given number of days.
+func (d *DB) PruneOldDeals(days int) error {
+	cutoff := time.Now().AddDate(0, 0, -days).UTC().Format(time.RFC3339)
+	_, err := d.db.Exec("DELETE FROM posted_deals WHERE posted_at < ?", cutoff)
+	return err
+}