fix(calendar): keep imported events with non-positive duration visible (#4484)

A single-day all-day event whose source writes DTEND equal to DTSTART
(treating DTEND as an inclusive bound rather than the RFC 5545 exclusive
one) was stored verbatim as a zero-duration row. list_events selects
events overlapping the window with `dtstart < end AND dtend > start`, so
that row is filtered out for any window starting at or after its date and
the event never appears, even though the import reported success.

Events created via the API never hit this because creation always
synthesizes a positive duration; only the two import paths can persist a
non-positive one. Clamp a non-positive end at import (import_ics and the
CalDAV pull) to the same default span used when DTEND is absent: one day
for all-day events, one hour otherwise.

Also repair the persisted state for users who already imported before this
clamp existed. Their stored zero-duration row is invisible, and re-importing
the same ICS hit the duplicate branch and skipped without touching it, so
the event stayed hidden. The duplicate branch now backfills the clamp onto
the matched row before skipping, and the response reports a `repaired` count.
(The CalDAV pull already rewrites dtend on re-sync, so it self-heals.)

routes/calendar_routes.py

@@ -34,6 +34,24 @@ def _ics_naive_dtstart(dt):
         return datetime(dt.year, dt.month, dt.day)
     return dt
 
+
+def _ensure_positive_duration(start_dt, end_dt, all_day):
+    """Clamp an imported event's end so it has a positive duration.
+
+    Some .ics exporters write a single-day all-day event with DTEND equal to
+    DTSTART (treating DTEND as inclusive rather than the RFC 5545 exclusive
+    bound). Stored verbatim that produces a zero-duration row, which the
+    list_events overlap filter (dtstart < end AND dtend > start) silently
+    drops — the event never appears on the calendar even though the web UI
+    would otherwise show it. Normalize a non-positive end to the same default
+    span used when DTEND is absent: one day for all-day events, one hour
+    otherwise.
+    """
+    if end_dt <= start_dt:
+        return start_dt + (timedelta(days=1) if all_day else timedelta(hours=1))
+    return end_dt
+
+
 # Single-user fallback identity. Used only when:
 #   1. The app is configured for single-user (no auth middleware), AND
 #   2. The request didn't resolve to an authenticated user.
@@ -1226,7 +1244,7 @@ def setup_calendar_routes() -> APIRouter:
                 db.commit()
                 db.refresh(target_cal)
 
-            imported = skipped = 0
+            imported = skipped = repaired = 0
             for comp in cal_data.walk():
                 if comp.name != "VEVENT":
                     continue
@@ -1262,6 +1280,18 @@ def setup_calendar_routes() -> APIRouter:
                         .first()
                     )
                     if existing:
+                        # An import predating the clamp below may have stored
+                        # this same event with a non-positive duration, which
+                        # the list_events overlap filter hides. Re-importing
+                        # lands here and would skip without touching that row,
+                        # so the event would stay invisible. Backfill the clamp
+                        # onto the stored row before skipping it.
+                        fixed_end = _ensure_positive_duration(
+                            existing.dtstart, existing.dtend, bool(existing.all_day)
+                        )
+                        if fixed_end != existing.dtend:
+                            existing.dtend = fixed_end
+                            repaired += 1
                         skipped += 1
                         continue
 
@@ -1295,6 +1325,8 @@ def setup_calendar_routes() -> APIRouter:
                     else:
                         end_dt = start_dt + timedelta(hours=1)
 
+                end_dt = _ensure_positive_duration(start_dt, end_dt, all_day)
+
                 ev = CalendarEvent(
                     uid=uid_val,
                     calendar_id=target_cal.id,
@@ -1315,6 +1347,7 @@ def setup_calendar_routes() -> APIRouter:
                 "ok": True,
                 "imported": imported,
                 "skipped": skipped,
+                "repaired": repaired,
                 "calendar": cal_display,
                 "calendar_id": target_cal.id,
             }