ci: add flake8 lint workflow; fix unused imports and f-string issues

Removes unused datetime/re imports (F401). Removes f prefix from
2 f-strings with no placeholders (F541). Fixes trailing whitespace
and missing newline (W291/W292). Config ignores style-only rules.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/lint.yml

@@ -0,0 +1,23 @@
+name: Lint
+
+on:
+  push:
+    branches: ["**"]
+  pull_request:
+    branches: ["**"]
+
+jobs:
+  python-lint:
+    name: Python (flake8)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Install Python and flake8
+        run: |
+          apt-get update -qq
+          apt-get install -y -qq python3 python3-pip
+          pip3 install flake8
+
+      - name: Run flake8
+        run: flake8 . --exclude=__pycache__,.git