feat: schedule rework

schema.sql

@@ -110,7 +110,14 @@ CREATE TABLE IF NOT EXISTS schedules (
     status TEXT NOT NULL DEFAULT 'pending', -- pending, accepted, declined, reschedule_requested
     description TEXT,
     reminder_sent BOOLEAN NOT NULL DEFAULT FALSE,
-    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    -- Reschedule negotiation: a proposed new time awaiting the counterpart's
+    -- response, kept separate from scheduled_at until accepted.
+    proposed_at TIMESTAMPTZ,
+    proposed_by BIGINT,
+    -- Snapshot of status before entering reschedule_requested, so a decline
+    -- knows whether to revert to 'pending' or 'accepted'.
+    pre_reschedule_status TEXT
 );
 
 CREATE TABLE IF NOT EXISTS schedule_messages (