fix: merge case-duplicate github repos and enforce lowercase url uniqueness

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

Author: mbani01

backend/src/osspckgs/migrations/V1784718693__merge_case_duplicate_github_repos.sql

@@ -0,0 +1,92 @@
+-- GitHub paths are case-insensitive, but repos.url is UNIQUE case-sensitively.
+-- Writers that predate URL lowercasing (cargo initial sync on 2026-06-19, maven
+-- enrichment before CM-1305) inserted mixed-case variants of rows that already
+-- existed lowercase: ~40k duplicate groups, ~10k of them serving duplicate
+-- security contacts from both variants. The CM-1305 backfills re-pointed
+-- package links to the lowercase rows but left the stale variants behind.
+--
+-- Keeper per group: the all-lowercase row when present (links were already
+-- re-pointed to it), else the lowest id. Only package_repos is primary data
+-- and gets re-pointed (repo_docker comes along since it is a free UPDATE);
+-- contacts, snapshots, and scorecard rows on losers are derived and re-fill
+-- via the regular sweeps, so they are dropped with the loser rows.
+
+CREATE TEMP TABLE repo_merge_members AS
+SELECT id AS repo_id, keeper_id, id = keeper_id AS is_keeper
+FROM (
+    SELECT
+        id,
+        FIRST_VALUE(id) OVER (
+            PARTITION BY LOWER(url)
+            ORDER BY (url = LOWER(url)) DESC, id
+        ) AS keeper_id,
+        COUNT(*) OVER (PARTITION BY LOWER(url)) AS group_size
+    FROM repos
+    WHERE host = 'github'
+) grouped
+WHERE group_size > 1;
+
+CREATE INDEX ON repo_merge_members (repo_id);
+ANALYZE repo_merge_members;
+
+-- Keep one link per (package, group): the keeper's if it exists, else the most
+-- recently verified. ROW_NUMBER instead of an EXISTS check against the keeper
+-- because 3-variant groups exist: two losers linking the same package would
+-- collide on UNIQUE (package_id, repo_id) after the re-point below.
+DELETE FROM package_repos
+WHERE id IN (
+    SELECT id
+    FROM (
+        SELECT pr.id,
+               ROW_NUMBER() OVER (
+                   PARTITION BY m.keeper_id, pr.package_id
+                   ORDER BY m.is_keeper DESC, pr.verified_at DESC, pr.id
+               ) AS rn
+        FROM package_repos pr
+        JOIN repo_merge_members m ON m.repo_id = pr.repo_id
+    ) ranked
+    WHERE rn > 1
+);
+
+UPDATE package_repos pr
+SET repo_id = m.keeper_id
+FROM repo_merge_members m
+WHERE pr.repo_id = m.repo_id
+  AND NOT m.is_keeper;
+
+UPDATE repo_docker d
+SET repo_id = m.keeper_id
+FROM repo_merge_members m
+WHERE d.repo_id = m.repo_id
+  AND NOT m.is_keeper;
+
+DELETE FROM repo_scorecard_checks c
+USING repo_merge_members m
+WHERE c.repo_id = m.repo_id
+  AND NOT m.is_keeper;
+
+-- Cascades security_contacts and repo_activity_snapshot on losers.
+DELETE FROM repos r
+USING repo_merge_members m
+WHERE r.id = m.repo_id
+  AND NOT m.is_keeper;
+
+-- Groups that had no lowercase variant: lowercase the surviving row. Safe
+-- against UNIQUE (url) — every row sharing LOWER(url) was in the same group,
+-- and its losers are gone by now.
+UPDATE repos r
+SET url = LOWER(r.url), updated_at = NOW()
+FROM repo_merge_members m
+WHERE r.id = m.repo_id
+  AND m.is_keeper
+  AND r.url <> LOWER(r.url);
+
+-- Recurrence guard; also fails the migration if any duplicate survived the
+-- merge. GitHub-only: it is the only host with both confirmed duplicates and
+-- guaranteed lowercase-on-write today (CASE_INSENSITIVE_HOSTS in
+-- canonicalizeRepoUrl also covers gitlab.com, whose merge is a follow-up).
+-- On other hosts case can be significant and writers do not normalize, so a
+-- wider index could reject legitimately distinct repos.
+CREATE UNIQUE INDEX IF NOT EXISTS repos_github_lower_url_uq
+    ON repos (LOWER(url))
+    WHERE host = 'github';