Fix mypy failure in Backend CI from django-stubs 6.0.3 upgrade

Two root causes introduced in quick succession during the 2026-04-23
batch of merges:

1. PR #1343 wired `Run mypy` into Backend CI with a per-module baseline
   in mypy.ini. The baseline captured every file that had type errors
   under django-stubs 6.0.2 (7208 errors across 357 files).
2. PR #1340 then bumped django-stubs 6.0.2 -> 6.0.3 in
   requirements/local.txt. 6.0.3's stricter ValuesQuerySet typing
   surfaced a new error on validate_v3_migration.py:303 -- a file that
   was type-clean under 6.0.2 and therefore NOT in the baseline.

Result: Backend CI `linter` job fails on main and every PR branched
off main.

Additionally, the mirrors-mypy pre-commit hook still pinned
django-stubs==6.0.2 while CI's `Run mypy` step installs 6.0.3 from
requirements/local.txt -- the exact hook/CI drift that the comment
above `additional_dependencies` warns against. This let the new error
slip past pre-commit.

Fix:
- validate_v3_migration.py: use `.values_list("content_hash", "count")`
  so the loop yields a typed tuple instead of relying on subscripting
  an annotated ValuesQuerySet (which 6.0.3 types as an annotated model
  rather than a dict). Functionally identical; zero behaviour change.
- .pre-commit-config.yaml: bump django-stubs pin to 6.0.3 so the hook
  and CI check against the same stub version again.

Author: JSv4

.pre-commit-config.yaml

@@ -61,7 +61,7 @@ repos:
         # major breakages (e.g. pypdf 7, openai 3) for the same reason.
         additional_dependencies:
           # --- Type stubs -----------------------------------------------
-          - django-stubs==6.0.2
+          - django-stubs==6.0.3
           - djangorestframework-stubs==3.16.9
           - types-requests==2.33.0.20260408
           - types-PyYAML==6.0.12.20260408

opencontractserver/documents/management/commands/validate_v3_migration.py

@@ -298,9 +298,11 @@ def _check_structural_set_uniqueness(self, verbose):
         )
 
         if verbose:
-            for dup in duplicates[:5]:
+            for content_hash, count in duplicates.values_list(
+                "content_hash", "count"
+            )[:5]:
                 self.stdout.write(
-                    f"    - Hash {dup['content_hash'][:16]}...: {dup['count']} duplicates"
+                    f"    - Hash {content_hash[:16]}...: {count} duplicates"
                 )
 
         return {"passed": False, "duplicate_hashes": duplicate_count}