fix: execute deduplication SQL independently to guarantee execution

Previous fix still had the deduplication inside the IF EXISTS (table) block.
If table exists, the deduplication would run, but it was still part of a DO
block which could fail silently.

Split into TWO separate Exec calls:
1. DO block for column additions (snapshot_date, backers_count)
2. Standalone DELETE for deduplication (runs EVERY time)

Also moved SET NOT NULL inside the IF NOT EXISTS block where it belongs
(only needed when first adding the column).

This ensures deduplication always runs before AutoMigrate creates the unique
index, regardless of database state.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Jing-yilin

backend/internal/db/db.go

@@ -160,6 +160,7 @@ func Init(cfg *config.Config) error {
 				) THEN
 					ALTER TABLE campaign_snapshots ADD COLUMN snapshot_date date;
 					UPDATE campaign_snapshots SET snapshot_date = DATE(snapshot_at);
+					ALTER TABLE campaign_snapshots ALTER COLUMN snapshot_date SET NOT NULL;
 				END IF;
 
 				-- Add backers_count if missing (added alongside snapshot_date).
@@ -169,21 +170,23 @@ func Init(cfg *config.Config) error {
 				) THEN
 					ALTER TABLE campaign_snapshots ADD COLUMN backers_count int DEFAULT 0;
 				END IF;
-
-				-- Set NOT NULL now that all rows have a value.
-				ALTER TABLE campaign_snapshots ALTER COLUMN snapshot_date SET NOT NULL;
 			END IF;
-
-			-- Deduplicate: keep only the latest snapshot per (campaign_pid, snapshot_date).
-			-- Run this EVERY time (not just when column is missing) to handle legacy duplicate data.
-			DELETE FROM campaign_snapshots a USING campaign_snapshots b
-			WHERE a.campaign_pid = b.campaign_pid
-			  AND a.snapshot_date = b.snapshot_date
-			  AND a.snapshot_at < b.snapshot_at;
 		END
 		$$;
 	`).Error; err != nil {
-		return fmt.Errorf("pre-migrate campaign_snapshots: %w", err)
+		return fmt.Errorf("pre-migrate campaign_snapshots columns: %w", err)
+	}
+
+	// Deduplicate snapshots: keep only the latest per (campaign_pid, snapshot_date).
+	// Run this EVERY time to handle legacy duplicate data from buggy upsert logic.
+	// Must run BEFORE AutoMigrate attempts to create the unique index.
+	if err := DB.Exec(`
+		DELETE FROM campaign_snapshots a USING campaign_snapshots b
+		WHERE a.campaign_pid = b.campaign_pid
+		  AND a.snapshot_date = b.snapshot_date
+		  AND a.snapshot_at < b.snapshot_at
+	`).Error; err != nil {
+		return fmt.Errorf("deduplicate campaign_snapshots: %w", err)
 	}
 
 	// NOW run AutoMigrate after all column renames are complete