fix: move snapshot deduplication outside IF block to handle existing duplicates

The deduplication logic was inside IF NOT EXISTS (snapshot_date), so it only
ran on fresh databases. Existing databases with duplicate data would fail to
create the unique index during AutoMigrate.

Move the DELETE statement outside the IF block so it runs on EVERY deployment,
ensuring duplicate data is cleaned up before AutoMigrate attempts to create
the unique index idx_campaign_snapshot_date.

Fixes deployment failure: ERROR: could not create unique index
"idx_campaign_snapshot_date" (SQLSTATE 23505)

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

Author: Jing-yilin

backend/internal/db/db.go

@@ -170,15 +170,16 @@ func Init(cfg *config.Config) error {
 					ALTER TABLE campaign_snapshots ADD COLUMN backers_count int DEFAULT 0;
 				END IF;
 
-				-- Deduplicate: keep only the latest snapshot per (campaign_pid, snapshot_date).
-				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;
-
 				-- 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 {