Render maven's own reminders to a calendar she owns (#127)

Radicale becomes a write-only render target, not a store. sqlite stays
canonical: every poll mavcaldav reads the pending reminders out of core and
publishes each one as a single-event iCal resource, withdrawing the ones that
have fired or been cancelled. Losing the collection costs nothing — the next
tick rebuilds it, and nothing is ever read back from it.

It structurally cannot write to a calendar maven only reads. The render URL and
credential are their own flags, and -render-url is refused at startup when it
names the collection -url reads; the only paths it addresses carry the
maven-reminder- prefix, so even aimed at the wrong collection it can only touch
resources it created. Rendering is off unless -render-url is given.

The calendar data model now lives in one place, internal/calendar: the Event,
the iCal parse it comes from and the render it goes to, the fact key/value
encoding, and the source constants that say which calendars may be written to.
It was a parse inlined in cmd/mavcaldav and a Sprintf in two files; #126 and
#128 both need to agree with it.

Fixes a latent day-boundary bug moved out of that inline parse: it took the day
number off a local clock reading but built the window boundaries in UTC, so on
a box east of Greenwich part of the evening fell outside "today" and the poller
saw an empty calendar after 20:00 UTC. Today is now the owner's day in the
owner's location, which is what the busy gate and the day plan mean.

internal/calendar/ical_render_test.go

@@ -0,0 +1,69 @@
+package calendar
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestRenderICalRoundTrips(t *testing.T) {
+	e := ReminderEvent(7, time.Date(2026, 8, 1, 18, 30, 0, 0, time.UTC), "позвонить маме", 0)
+	if e.UID != "maven-reminder-7" {
+		t.Errorf("UID = %q", e.UID)
+	}
+	if got := e.End.Sub(e.Start); got != DefaultReminderDuration {
+		t.Errorf("duration = %v, want %v", got, DefaultReminderDuration)
+	}
+	if got, want := ReminderPath(7), "maven-reminder-7.ics"; got != want {
+		t.Errorf("ReminderPath = %q, want %q", got, want)
+	}
+
+	body := RenderICal([]Event{e})
+	if !strings.HasPrefix(body, "BEGIN:VCALENDAR\r\n") || !strings.HasSuffix(body, "END:VCALENDAR\r\n") {
+		t.Fatalf("not a VCALENDAR body:\n%s", body)
+	}
+
+	back := ParseICal([]byte(body), e.Start.Add(-time.Hour), e.Start.Add(time.Hour))
+	if len(back) != 1 {
+		t.Fatalf("got %d events back, want 1:\n%s", len(back), body)
+	}
+	if back[0].UID != e.UID || back[0].Summary != e.Summary {
+		t.Errorf("round trip lost identity: %+v", back[0])
+	}
+	if !back[0].Start.Equal(e.Start) || !back[0].End.Equal(e.End) {
+		t.Errorf("round trip lost times: %+v", back[0])
+	}
+}
+
+func TestRenderICalIsDeterministic(t *testing.T) {
+	e := ReminderEvent(1, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "выпить воды", 0)
+	if RenderICal([]Event{e}) != RenderICal([]Event{e}) {
+		t.Error("the same reminder must render byte-identically, or every poll re-PUTs it")
+	}
+}
+
+// A reminder payload is owner-supplied text. It must not be able to close the
+// VEVENT and inject properties of its own.
+func TestRenderICalEscapesInjection(t *testing.T) {
+	e := ReminderEvent(2, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC),
+		"обед\r\nEND:VEVENT\r\nBEGIN:VEVENT\r\nSUMMARY:injected", 0)
+	body := RenderICal([]Event{e})
+	// Count line-initial occurrences: the escaped text still contains the
+	// characters "BEGIN:VEVENT", it just can no longer start a line.
+	if n := strings.Count(body, "\r\nBEGIN:VEVENT\r\n"); n != 1 {
+		t.Fatalf("payload injected a second VEVENT (%d):\n%s", n, body)
+	}
+	if n := strings.Count(body, "\r\nEND:VEVENT\r\n"); n != 1 {
+		t.Fatalf("payload closed the VEVENT early (%d):\n%s", n, body)
+	}
+	if !strings.Contains(body, `SUMMARY:обед\nEND:VEVENT`) {
+		t.Errorf("newlines should be escaped, not dropped:\n%s", body)
+	}
+}
+
+func TestReminderEventEmptyPayload(t *testing.T) {
+	e := ReminderEvent(3, time.Date(2026, 8, 1, 9, 0, 0, 0, time.UTC), "   ", 0)
+	if e.Summary != "напоминание" {
+		t.Errorf("Summary = %q, want the neutral RU fallback", e.Summary)
+	}
+}