fix: cleanup maintainers duplicates and prevent them [CM-677] (#3971)

Signed-off-by: Mouad BANI <mouad-mb@outlook.com>

Author: mbani01

backend/src/database/migrations/U1774628604__fixMaintainersInternalUniqueIndex.sql

@@ -0,0 +1,32 @@
+-- Fix maintainersInternal unique index and clean up duplicates.
+--
+-- Problem:
+--   The unique index on ("repoId", "identityId", "startDate", "endDate") fails to
+--   prevent duplicates because startDate and endDate are NULL on insert, and
+--   PostgreSQL treats NULL != NULL in unique indexes — so the constraint never fires.
+--
+-- Fix:
+--   Unique index on ("repoId", "identityId", role). All three columns are NOT NULL,
+--   so the uniqueness check always works. startDate/endDate become purely informational.
+
+-- Step 1: Remove duplicate rows, keeping the most recently updated row per group.
+DELETE FROM "maintainersInternal"
+WHERE id IN (
+    SELECT id FROM (
+        SELECT
+            id,
+            ROW_NUMBER() OVER (
+                PARTITION BY "repoId", "identityId", role
+                ORDER BY "updatedAt" DESC NULLS LAST, "createdAt" DESC NULLS LAST
+            ) AS rn
+        FROM "maintainersInternal"
+    ) ranked
+    WHERE rn > 1
+);
+
+-- Step 2: Drop the existing unique index.
+DROP INDEX IF EXISTS maintainers_internal_repo_identity_unique_idx;
+
+-- Step 3: Create the correct unique index on non-nullable columns.
+CREATE UNIQUE INDEX maintainers_internal_repo_identity_role_unique_idx
+    ON "maintainersInternal" ("repoId", "identityId", role);

backend/src/database/migrations/V1774628604__fixMaintainersInternalUniqueIndex.sql

@@ -339,8 +339,12 @@ async def upsert_maintainer(
         INSERT INTO "maintainersInternal"
         ("role", "originalRole", "repoUrl", "repoId", "identityId", "startDate", "endDate")
         VALUES ($1, $2, $3, $4, $5, $6, $7)
-        ON CONFLICT ("repoId", "identityId", "startDate", "endDate") DO UPDATE
-        SET role = EXCLUDED.role, "originalRole" = EXCLUDED."originalRole", "updatedAt" = NOW()
+        ON CONFLICT ("repoId", "identityId", role) DO UPDATE
+        SET "originalRole" = EXCLUDED."originalRole",
+            "repoUrl" = EXCLUDED."repoUrl",
+            "startDate" = COALESCE("maintainersInternal"."startDate", EXCLUDED."startDate"),
+            "endDate" = NULL,
+            "updatedAt" = NOW()
     """
     await execute(
         sql_query,