Add calendar_event_participants table and dedup index for PST re-imports

src-tauri/migrations/0005_calendar_attendees.sql

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+-- WhispAssist calendar-event attendees schema (Phase 6, T6.1/T6.2, FR-CAL-1).
+-- Forward-only migration. Mirrors docs/03-data-model.md.
+PRAGMA foreign_keys = ON;
+
+-- Attendees per imported calendar event (populated at import time from the
+-- source's own attendee list, e.g. a .pst appointment's ATTENDEE lines) —
+-- distinct from meeting_participants, which links a *recording* to people
+-- once a meeting is attached to an event (T6.3/T6.6).
+CREATE TABLE calendar_event_participants (
+    calendar_event_id TEXT NOT NULL REFERENCES calendar_events(id) ON DELETE CASCADE,
+    participant_id     TEXT NOT NULL REFERENCES participants(id),
+    role                TEXT,                       -- organizer|required|optional
+    PRIMARY KEY (calendar_event_id, participant_id)
+);
+
+-- Dedup re-imports of the same PST (or the same event across multiple PSTs
+-- from the same source kind) by the source's own stable UID.
+CREATE UNIQUE INDEX idx_calendar_events_source_uid ON calendar_events(source, raw_uid)
+    WHERE raw_uid IS NOT NULL;