feat(calendar): expand recurring events in GET /calendar and /upcoming

expandRecurringEvents() iterates from the event's original start date,
generating all occurrences within the requested window using the existing
nextOccurrence() service (max 1000 iterations). The SQL query is extended
to also fetch recurring events that started before the window. Event
duration is preserved across instances. Virtual instances carry
is_recurring_instance=1 and are shown with a repeat icon in the agenda
view. /upcoming expands across a 90-day forward window.

Closes BL-01.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Calendar: recurring events are now expanded in GET /api/v1/calendar — all occurrences within the requested date window are returned as virtual instances; duration is preserved; instances are marked with is_recurring_instance=1 and shown with a ↻ icon in the agenda view; /upcoming also expands recurring events within a 90-day window
 - Budget: recurring entries auto-generate instances for each viewed month; instances deleted by the user are skipped permanently via `budget_recurrence_skipped` table; generated instances are marked with ↩ in the transaction list
 - Budget: month-over-month comparison in summary cards — each card (Einnahmen, Ausgaben, Saldo) shows a trend line (▲/▼ + delta amount vs. previous month); previous month summary is fetched in parallel with current month