feat(db): add platform columns to reschedule_proposals

src/GmRelay.Bot/Migrations/V018__discord_reschedule_proposals.sql

@@ -0,0 +1,19 @@
+-- =============================================================
+-- V018: Add platform columns to reschedule_proposals
+-- =============================================================
+-- Add platform columns to reschedule_proposals to support Discord reschedule voting.
+-- proposed_by is made nullable so Discord proposals can leave it NULL
+-- (Discord snowflakes don't fit in BIGINT safely).
+-- =============================================================
+
+ALTER TABLE reschedule_proposals
+    ALTER COLUMN proposed_by DROP NOT NULL;
+
+ALTER TABLE reschedule_proposals
+    ADD COLUMN source_platform VARCHAR(50),
+    ADD COLUMN proposed_by_external_user_id VARCHAR(255);
+
+UPDATE reschedule_proposals
+SET source_platform = 'Telegram',
+    proposed_by_external_user_id = proposed_by::TEXT
+WHERE source_platform IS NULL;