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>

go.mod

@@ -0,0 +1,36 @@
+module github.com/prosolis/Pastel
+
+go 1.25.0
+
+require (
+	github.com/jmoiron/sqlx v1.4.0
+	github.com/joho/godotenv v1.5.1
+	maunium.net/go/mautrix v0.26.3
+	modernc.org/sqlite v1.46.1
+)
+
+require (
+	filippo.io/edwards25519 v1.1.0 // indirect
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/mattn/go-colorable v0.1.14 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-sqlite3 v1.14.34 // indirect
+	github.com/ncruces/go-strftime v1.0.0 // indirect
+	github.com/petermattis/goid v0.0.0-20260113132338-7c7de50cc741 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/rs/zerolog v1.34.0 // indirect
+	github.com/tidwall/gjson v1.18.0 // indirect
+	github.com/tidwall/match v1.1.1 // indirect
+	github.com/tidwall/pretty v1.2.1 // indirect
+	github.com/tidwall/sjson v1.2.5 // indirect
+	go.mau.fi/util v0.9.6 // indirect
+	golang.org/x/crypto v0.48.0 // indirect
+	golang.org/x/exp v0.0.0-20260212183809-81e46e3db34a // indirect
+	golang.org/x/net v0.50.0 // indirect
+	golang.org/x/sys v0.41.0 // indirect
+	golang.org/x/text v0.34.0 // indirect
+	modernc.org/libc v1.67.6 // indirect
+	modernc.org/mathutil v1.7.1 // indirect
+	modernc.org/memory v1.11.0 // indirect
+)