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/watchlist/watchlist.go

@@ -0,0 +1,170 @@
+package watchlist
+
+import (
+	"strings"
+	"time"
+	"unicode"
+
+	"github.com/jmoiron/sqlx"
+)
+
+const watchDuration = 180 * 24 * time.Hour
+
+// WatchEntry represents a single watchlist entry.
+type WatchEntry struct {
+	ID                 int64     `db:"id"`
+	UserID             string    `db:"user_id"`
+	GameName           string    `db:"game_name"`
+	GameNameNormalized string    `db:"game_name_normalized"`
+	CreatedAt          time.Time `db:"created_at"`
+	ExpiresAt          time.Time `db:"expires_at"`
+	ExpiryWarned       int       `db:"expiry_warned"`
+}
+
+// Match represents a user whose watchlist entry matched a deal.
+type Match struct {
+	UserID   string
+	GameName string // original user-provided name for display
+}
+
+// Store handles watchlist database operations.
+type Store struct {
+	db *sqlx.DB
+}
+
+// NewStore creates a new watchlist store.
+func NewStore(db *sqlx.DB) *Store {
+	return &Store{db: db}
+}
+
+// Normalize lowercases, strips non-alphanumeric (keeping spaces), and collapses whitespace.
+func Normalize(s string) string {
+	s = strings.ToLower(s)
+	var b strings.Builder
+	for _, r := range s {
+		if unicode.IsLetter(r) || unicode.IsDigit(r) || r == ' ' {
+			b.WriteRune(r)
+		}
+	}
+	return strings.Join(strings.Fields(b.String()), " ")
+}
+
+// AddWatch adds a game to a user's watchlist. Returns false if already watched.
+func (s *Store) AddWatch(userID, gameName string) (bool, error) {
+	normalized := Normalize(gameName)
+	if normalized == "" {
+		return false, nil
+	}
+	expiresAt := time.Now().Add(watchDuration).UTC().Format(time.RFC3339)
+	result, err := s.db.Exec(
+		`INSERT OR IGNORE INTO watchlist (user_id, game_name, game_name_normalized, expires_at)
+		 VALUES (?, ?, ?, ?)`,
+		userID, gameName, normalized, expiresAt,
+	)
+	if err != nil {
+		return false, err
+	}
+	rows, _ := result.RowsAffected()
+	return rows > 0, nil
+}
+
+// RemoveWatch removes a game from a user's watchlist. Returns false if not found.
+func (s *Store) RemoveWatch(userID, gameName string) (bool, error) {
+	normalized := Normalize(gameName)
+	result, err := s.db.Exec(
+		"DELETE FROM watchlist WHERE user_id = ? AND game_name_normalized = ?",
+		userID, normalized,
+	)
+	if err != nil {
+		return false, err
+	}
+	rows, _ := result.RowsAffected()
+	return rows > 0, nil
+}
+
+// ExtendWatch resets the expiry to 180 days from now. Returns false if not found.
+func (s *Store) ExtendWatch(userID, gameName string) (bool, error) {
+	normalized := Normalize(gameName)
+	expiresAt := time.Now().Add(watchDuration).UTC().Format(time.RFC3339)
+	result, err := s.db.Exec(
+		`UPDATE watchlist SET expires_at = ?, expiry_warned = 0
+		 WHERE user_id = ? AND game_name_normalized = ?`,
+		expiresAt, userID, normalized,
+	)
+	if err != nil {
+		return false, err
+	}
+	rows, _ := result.RowsAffected()
+	return rows > 0, nil
+}
+
+// ListWatches returns all active watches for a user.
+func (s *Store) ListWatches(userID string) ([]WatchEntry, error) {
+	var entries []WatchEntry
+	err := s.db.Select(&entries,
+		`SELECT id, user_id, game_name, game_name_normalized, created_at, expires_at, expiry_warned
+		 FROM watchlist WHERE user_id = ? AND expires_at > CURRENT_TIMESTAMP
+		 ORDER BY created_at`,
+		userID,
+	)
+	return entries, err
+}
+
+// FindMatchingUsers finds all users whose watchlist entries match the given deal title.
+// Loads all active entries and checks normalized substring containment in Go.
+func (s *Store) FindMatchingUsers(dealTitle string) ([]Match, error) {
+	normalizedTitle := Normalize(dealTitle)
+
+	var entries []WatchEntry
+	err := s.db.Select(&entries,
+		`SELECT user_id, game_name, game_name_normalized
+		 FROM watchlist WHERE expires_at > CURRENT_TIMESTAMP`,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	var matches []Match
+	for _, e := range entries {
+		if strings.Contains(normalizedTitle, e.GameNameNormalized) {
+			matches = append(matches, Match{
+				UserID:   e.UserID,
+				GameName: e.GameName,
+			})
+		}
+	}
+	return matches, nil
+}
+
+// GetExpiringWatches returns entries expiring within the given number of days
+// that haven't been warned yet.
+func (s *Store) GetExpiringWatches(withinDays int) ([]WatchEntry, error) {
+	deadline := time.Now().AddDate(0, 0, withinDays).UTC().Format(time.RFC3339)
+	now := time.Now().UTC().Format(time.RFC3339)
+
+	var entries []WatchEntry
+	err := s.db.Select(&entries,
+		`SELECT id, user_id, game_name, game_name_normalized, created_at, expires_at, expiry_warned
+		 FROM watchlist
+		 WHERE expires_at > ? AND expires_at <= ? AND expiry_warned = 0`,
+		now, deadline,
+	)
+	return entries, err
+}
+
+// MarkExpiryWarned sets the expiry_warned flag on an entry.
+func (s *Store) MarkExpiryWarned(id int64) error {
+	_, err := s.db.Exec("UPDATE watchlist SET expiry_warned = 1 WHERE id = ?", id)
+	return err
+}
+
+// PurgeExpired deletes entries past their expiry date. Returns count deleted.
+func (s *Store) PurgeExpired() (int, error) {
+	now := time.Now().UTC().Format(time.RFC3339)
+	result, err := s.db.Exec("DELETE FROM watchlist WHERE expires_at <= ?", now)
+	if err != nil {
+		return 0, err
+	}
+	rows, _ := result.RowsAffected()
+	return int(rows), nil
+}