feat(events): replace end_dt column with duration_minutes (#160)

Structural fix for the "end before start" bug class observed on prod
2026-04-29. Bad data became inexpressible at the schema level instead
of getting trapped in defensive read-path filters.

The hotfix that landed earlier today (94b169f) is reverted by the
preceding revert commit; this commit supersedes it cleanly with a
proper data-model change.

## Schema (migration 0043)

- Add `duration_minutes INTEGER NULLABLE` column on `events`.
- CHECK constraint: ``duration_minutes IS NULL OR duration_minutes >= 0``.
- Backfill from existing `end_dt`:
    - end_dt valid (end > start) → duration_minutes = total minutes
    - end_dt == start → duration_minutes = 0 (zero-duration point)
    - end_dt NULL or end_dt < start → duration_minutes = NULL
      (the corrupt prod row collapses cleanly to a point event)
- Drop the `end_dt` column. The wire format is preserved — `to_dict()`
  emits `end_dt` as a derived `start_dt + duration_minutes`. Existing
  API consumers (Flutter app, web frontend, CalDAV sync) keep
  receiving the same response shape; they just no longer have a way
  to PUT a stored `end_dt` that disagrees with `start_dt`.

## Service layer

- `Event.end_dt` becomes a `@property`. Setting it would require a
  setter we deliberately don't define — writes always go through
  `duration_minutes`.
- `_normalize_duration` is the single source-of-truth for input
  reduction. Accepts (start, end_dt, duration_minutes), returns the
  canonical `duration_minutes`, raises `ValueError` for negative
  durations, end-before-start, or end/duration disagreement.
- `create_event` and `update_event` accept either `end_dt` or
  `duration_minutes` for ergonomic compat; both convert via
  `_normalize_duration`. Update validates the post-update state when
  the patch includes either.
- `list_events` filter is simpler now: a coarse SQL prefilter
  (`start_dt <= date_to`) plus Python-side refinement using the
  derived `end_dt`. Avoids Postgres-specific interval arithmetic in
  the WHERE clause; refinement runs over a per-user result set so
  there's no scan-cost concern at personal scale.
- Recurring-event expansion uses `event.duration_minutes` directly
  instead of computing `end - start`. No more negative-timedelta
  hazard.

## CalDAV sync (incoming + outgoing)

- `caldav_sync.py` (pull) and `calendar_sync.py` (Radicale upsert)
  both convert iCal `DTEND` → `duration_minutes` on the way in.
  Outbound iCal still emits `DTEND` as `start_dt + duration_minutes`
  via the model's derived property. iCal interop is unchanged.

## Behavioral upgrade for `update_event`

Pure end_dt model: moving start past the existing end_dt would either
silently corrupt or hard-reject. Duration model: the duration is
preserved by default, so moving start slides the effective end
forward — which is what users mean when they "move" an event.
Explicit clear is still possible via `end_dt=None`.

## Tests

`tests/test_events_service.py`:
- 6 new `_normalize_duration` unit tests (sugar conversion, zero
  duration valid as point event, end-before-start rejected, negative
  duration rejected, inconsistent end+duration rejected, none → None)
- New behavioral test: `update_event` preserves duration when only
  start_dt changes (sliding semantics)
- New: clearing `end_dt=None` on update collapses to point event
- New: list_events surfaces a point event in the upcoming window
- New: list_events excludes a timed event whose effective end has
  already passed
- Existing mock-event helper updated to use `duration_minutes`
  instead of stored `end_dt`.

44 event-related tests pass; ruff clean.

## Out of scope (separate task)

Fable #161 — `find_events_by_query` returning multiple matches and
silently picking matches[0]. The exact root cause of how event id=2
got mutated in the first place; orthogonal to the storage model.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/routes/events.py

@@ -54,19 +54,23 @@ async def create_event():
         end_dt = _parse_dt(data["end_dt"]) if data.get("end_dt") else None
     except ValueError:
         return jsonify({"error": "Invalid datetime format"}), 400
-    event = await events_svc.create_event(
-        user_id=_get_current_user_id(),
-        title=data["title"],
-        start_dt=start_dt,
-        end_dt=end_dt,
-        all_day=data.get("all_day", False),
-        description=data.get("description", ""),
-        location=data.get("location", ""),
-        color=data.get("color", ""),
-        recurrence=data.get("recurrence"),
-        project_id=data.get("project_id"),
-        reminder_minutes=data.get("reminder_minutes"),
-    )
+    try:
+        event = await events_svc.create_event(
+            user_id=_get_current_user_id(),
+            title=data["title"],
+            start_dt=start_dt,
+            end_dt=end_dt,
+            duration_minutes=data.get("duration_minutes"),
+            all_day=data.get("all_day", False),
+            description=data.get("description", ""),
+            location=data.get("location", ""),
+            color=data.get("color", ""),
+            recurrence=data.get("recurrence"),
+            project_id=data.get("project_id"),
+            reminder_minutes=data.get("reminder_minutes"),
+        )
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
     return jsonify(event.to_dict()), 201
 
 
@@ -93,7 +97,7 @@ async def update_event(event_id: int):
     for bool_field in ("all_day",):
         if bool_field in data:
             fields[bool_field] = data[bool_field]
-    for int_field in ("project_id", "reminder_minutes"):
+    for int_field in ("project_id", "reminder_minutes", "duration_minutes"):
         if int_field in data:
             fields[int_field] = data[int_field]
     for dt_field in ("start_dt", "end_dt"):
@@ -106,11 +110,14 @@ async def update_event(event_id: int):
                     fields[dt_field] = _parse_dt(data[dt_field])
                 except ValueError:
                     return jsonify({"error": f"Invalid datetime for {dt_field}"}), 400
-    event = await events_svc.update_event(
-        user_id=_get_current_user_id(),
-        event_id=event_id,
-        **fields,
-    )
+    try:
+        event = await events_svc.update_event(
+            user_id=_get_current_user_id(),
+            event_id=event_id,
+            **fields,
+        )
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
     if event is None:
         return jsonify({"error": "Event not found"}), 404
     return jsonify(event.to_dict())