feat(weather)!: add /thuyvan flood alerts for Tân Thuận

/thuyvan shows the 5-day tide-peak forecast at Phú An and Nhà Bè from
the Đài KTTV Nam Bộ bulletin PDF, the Open-Meteo rain forecast, and live
VNDMS river gauges within 30 km. /thuyvan_subscribe opts a chat into a
10:30 ICT push, retried at 12:30, sent only when a forecast peak reaches
báo động I (1.40 m) or a day's rain reaches 50 mm. A missing or stale
bulletin fails the push instead of reading as no risk.

The thoitiet module is renamed to weather; the /thoitiet* commands keep
their names.

BREAKING CHANGE: the module key is now "weather". A MODULES list that
names thoitiet fails at startup and must name weather instead.

internal/modules/weather/format.go

@@ -0,0 +1,174 @@
+package weather
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// sourceLine credits the data source, as Open-Meteo's CC BY 4.0 licence asks.
+const sourceLine = "Nguồn: Open-Meteo"
+
+type condition struct {
+	label string
+	emoji string
+}
+
+// wmoConditions maps WMO weather interpretation codes, the "weather_code"
+// field Open-Meteo returns, to a Vietnamese label.
+var wmoConditions = map[int]condition{
+	0:  {"Trời quang", "☀️"},
+	1:  {"Ít mây", "🌤️"},
+	2:  {"Mây rải rác", "⛅"},
+	3:  {"Nhiều mây", "☁️"},
+	45: {"Sương mù", "🌫️"},
+	48: {"Sương mù đóng băng", "🌫️"},
+	51: {"Mưa phùn nhẹ", "🌦️"},
+	53: {"Mưa phùn", "🌦️"},
+	55: {"Mưa phùn dày", "🌧️"},
+	56: {"Mưa phùn băng nhẹ", "🌧️"},
+	57: {"Mưa phùn băng", "🌧️"},
+	61: {"Mưa nhẹ", "🌧️"},
+	63: {"Mưa vừa", "🌧️"},
+	65: {"Mưa to", "🌧️"},
+	66: {"Mưa băng nhẹ", "🌧️"},
+	67: {"Mưa băng", "🌧️"},
+	71: {"Tuyết nhẹ", "🌨️"},
+	73: {"Tuyết vừa", "🌨️"},
+	75: {"Tuyết dày", "❄️"},
+	77: {"Tuyết hạt", "🌨️"},
+	80: {"Mưa rào nhẹ", "🌦️"},
+	81: {"Mưa rào", "🌦️"},
+	82: {"Mưa rào rất to", "⛈️"},
+	85: {"Mưa tuyết nhẹ", "🌨️"},
+	86: {"Mưa tuyết", "🌨️"},
+	95: {"Dông", "⛈️"},
+	96: {"Dông kèm mưa đá nhẹ", "⛈️"},
+	99: {"Dông kèm mưa đá", "⛈️"},
+}
+
+func conditionFor(code int) condition {
+	if c, ok := wmoConditions[code]; ok {
+		return c
+	}
+	return condition{"Không rõ", "🌡️"}
+}
+
+// vnWeekdays are the short Vietnamese weekday names, indexed by time.Weekday.
+var vnWeekdays = [...]string{"CN", "T2", "T3", "T4", "T5", "T6", "T7"}
+
+// hourlyViewHours is how many upcoming hours /thoitiet lists.
+const hourlyViewHours = 6
+
+// formatHourly renders current conditions followed by one block per upcoming
+// hour.
+func formatHourly(p place, f forecast) string {
+	c := f.Current
+	cur := conditionFor(c.WeatherCode)
+	h := f.Hourly
+	idx := f.upcomingHours(hourlyViewHours)
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "🕐 Thời tiết %d giờ tới — %s\n", len(idx), displayName(p))
+	fmt.Fprintf(&sb, "Hiện tại %s: %s (cảm giác %s), %s %s\n", clock(c.Time), temp(c.Temperature),
+		temp(c.ApparentTemperature), cur.label, cur.emoji)
+	for _, i := range idx {
+		hc := conditionFor(h.WeatherCode[i])
+		fmt.Fprintf(&sb, "\n%s %s %s, %s (cảm giác %s)\n", clock(h.Time[i]), hc.emoji, hc.label,
+			temp(h.Temperature[i]), temp(h.ApparentTemperature[i]))
+		fmt.Fprintf(&sb, "Mưa %d%% (%s mm), độ ẩm %d%%, gió %s km/h\n", round(h.PrecipitationProbability[i]),
+			decimal(h.Precipitation[i]), round(h.Humidity[i]), decimal(h.WindSpeed[i]))
+	}
+	sb.WriteString("\n" + sourceLine)
+	return sb.String()
+}
+
+// formatToday renders current conditions plus today's daily summary.
+func formatToday(p place, f forecast) string {
+	c := f.Current
+	cur := conditionFor(c.WeatherCode)
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "%s Thời tiết hôm nay %s — %s\n", conditionFor(f.Daily.WeatherCode[0]).emoji,
+		shortDate(f.Daily.Time[0]), displayName(p))
+	fmt.Fprintf(&sb, "Hiện tại: %s (cảm giác %s), %s %s\n", temp(c.Temperature), temp(c.ApparentTemperature),
+		cur.label, cur.emoji)
+	fmt.Fprintf(&sb, "Độ ẩm %d%%, gió %s km/h\n", round(c.Humidity), decimal(c.WindSpeed))
+	writeDay(&sb, "Cả ngày", f.Daily, 0)
+	sb.WriteString(sourceLine)
+	return sb.String()
+}
+
+// formatTomorrow renders tomorrow's daily summary.
+func formatTomorrow(p place, f forecast) string {
+	d := f.Daily
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "%s Thời tiết ngày mai %s %s — %s\n", conditionFor(d.WeatherCode[1]).emoji,
+		weekday(d.Time[1]), shortDate(d.Time[1]), displayName(p))
+	writeDay(&sb, "Dự báo", d, 1)
+	sb.WriteString(sourceLine)
+	return sb.String()
+}
+
+// formatWeek renders one line per day for every day in the forecast.
+func formatWeek(p place, f forecast) string {
+	d := f.Daily
+	n := d.days()
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "📅 Thời tiết %d ngày tới — %s\n", n, displayName(p))
+	for i := range n {
+		c := conditionFor(d.WeatherCode[i])
+		fmt.Fprintf(&sb, "%s %s: %s–%s %s %s, mưa %d%%\n", weekday(d.Time[i]), shortDate(d.Time[i]),
+			tempValue(d.TemperatureMin[i]), temp(d.TemperatureMax[i]), c.emoji, c.label,
+			round(d.PrecipitationProbability[i]))
+	}
+	sb.WriteString(sourceLine)
+	return sb.String()
+}
+
+// writeDay renders the daily lines shared by the today and tomorrow views.
+func writeDay(sb *strings.Builder, label string, d dailyWeather, i int) {
+	c := conditionFor(d.WeatherCode[i])
+	fmt.Fprintf(sb, "%s: %s–%s, %s %s\n", label, tempValue(d.TemperatureMin[i]), temp(d.TemperatureMax[i]),
+		c.label, c.emoji)
+	fmt.Fprintf(sb, "Khả năng mưa %d%% (%s mm), UV %s\n", round(d.PrecipitationProbability[i]),
+		decimal(d.PrecipitationSum[i]), decimal(d.UVIndexMax[i]))
+	fmt.Fprintf(sb, "Mặt trời mọc %s, lặn %s\n", clock(d.Sunrise[i]), clock(d.Sunset[i]))
+}
+
+func round(v float64) int { return int(math.Round(v)) }
+
+func tempValue(v float64) string { return strconv.Itoa(round(v)) }
+
+func temp(v float64) string { return tempValue(v) + "°C" }
+
+// decimal renders one decimal place with the Vietnamese comma separator.
+func decimal(v float64) string {
+	return strings.Replace(strconv.FormatFloat(v, 'f', 1, 64), ".", ",", 1)
+}
+
+// shortDate turns Open-Meteo's "2026-10-01" into "01/10".
+func shortDate(iso string) string {
+	t, err := time.Parse(time.DateOnly, iso)
+	if err != nil {
+		return iso
+	}
+	return t.Format("02/01")
+}
+
+func weekday(iso string) string {
+	t, err := time.Parse(time.DateOnly, iso)
+	if err != nil {
+		return ""
+	}
+	return vnWeekdays[t.Weekday()]
+}
+
+// clock turns Open-Meteo's local "2026-10-01T05:42" into "05:42".
+func clock(iso string) string {
+	t, err := time.Parse("2006-01-02T15:04", iso)
+	if err != nil {
+		return iso
+	}
+	return t.Format("15:04")
+}