feat: enforce 85% test coverage in CI and fix cross-platform strftime bug

Add pyproject.toml with pytest-cov config so CI fails if coverage drops
below 85%. Fix series_view _format_start using Linux-only %-I/%-d codes
that crash on Windows — use manual formatting instead.

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

pyproject.toml

@@ -0,0 +1,15 @@
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+addopts = "-v --tb=short"
+
+[tool.coverage.run]
+source = ["app"]
+
+[tool.coverage.report]
+fail_under = 85
+show_missing = true
+skip_empty = true
+exclude_lines = [
+    "pragma: no cover",
+    "if __name__ == .__main__.",
+]