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.)

src/caldav_sync.py

@@ -274,6 +274,7 @@ def _sync_blocking(owner: str, url: str, username: str, password: str, account_i
     # the integrations form still works, sync just no-ops with an error.
     from caldav.lib.error import AuthorizationError, NotFoundError
     from core.database import CalendarCal, CalendarEvent, SessionLocal
+    from routes.calendar_routes import _ensure_positive_duration
 
     result = {"calendars": 0, "events": 0, "deleted": 0, "errors": []}
 
@@ -390,6 +391,11 @@ def _sync_blocking(owner: str, url: str, username: str, password: str, account_i
                             end_dt = start_dt + timedelta(days=1)
                         else:
                             end_dt = start_dt + timedelta(hours=1)
+                        # A synced event with DTEND <= DTSTART (e.g. a single-day
+                        # all-day event whose source wrote DTEND equal to DTSTART)
+                        # would be stored zero-duration and silently dropped by the
+                        # list_events overlap filter. Clamp to a positive span.
+                        end_dt = _ensure_positive_duration(start_dt, end_dt, all_day)
 
                         # is_utc reflects whether the source carried a TZ
                         # we converted from. All-day = no TZ semantics.