ci: mypy ratchet -- keep type-clean modules clean (71 modules, 536 -> 442 errors) (#661)

* ci: mypy ratchet -- keep type-clean modules clean

mypy-clean.txt lists the 71 modules under src/ that type-check clean;
scripts/check_types.py runs mypy (--follow-imports=silent) on exactly
those files and fails on any error or a missing/unsorted/duplicate entry.
A new "Type check (mypy ratchet)" CI job runs it with mypy 1.20.2 and
pinned stubs; the manual pre-commit mypy hook now runs the same script
(a local hook, so mypy sees the installed requirements like CI does).

35 modules were made clean with annotation-only fixes: hints, typing.cast,
TYPE_CHECKING imports, implicit-Optional defaults made explicit, and
annotations widened (never guards removed) where mypy called a defensive
isinstance check unreachable. No runtime behaviour change.

mypy.ini: numpy and orjson are treated as Any (follow_imports=skip, also
for stubs). numpy 2.3+ stubs use 3.12 `type` statements that mypy won't
parse at python_version 3.10, and orjson is optional, so seeing its stubs
made the result depend on whether it was installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* chore: annotate check_types.py's list-form mypy subprocess

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/test.yml

@@ -112,3 +112,31 @@ jobs:
           BASE: http://127.0.0.1:5000
           REQUIRE_DOM: "1"
         run: node test/js/run_all.js
+
+  type-check:
+    name: Type check (mypy ratchet)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+
+      - uses: actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b # v5.3.0
+        with:
+          python-version: "3.12"
+          cache: pip
+
+      # The runtime requirements are installed so mypy sees the real types of
+      # PIL, requests, psutil and friends -- missing, they'd be Any and the
+      # result would differ from a developer's machine. mypy and the stubs are
+      # pinned so a new release can't turn this red without a code change.
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt -r web_interface/requirements.txt
+          pip install mypy==1.20.2 types-requests==2.33.0.20260906 types-pytz==2026.4.0.20260926
+
+      # mypy on exactly the modules in mypy-clean.txt; fails on any error in
+      # them, or if a listed file is missing. See CONTRIBUTING.md.
+      - name: Run mypy on the ratchet list
+        run: python scripts/check_types.py