feat: pytest-cov configuration and test audit cleanup

Add full pytest-cov configuration with fail_under=60% threshold,
HTML/XML report targets, and proper exclude_lines. Fix websocket
history test to use public broadcast() API instead of manually
manipulating internals. Audit confirmed 491 tests at 71.2% coverage.

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

pyproject.toml

@@ -83,4 +83,27 @@ addopts = "-v --tb=short"
 
 [tool.coverage.run]
 source = ["src"]
-omit = ["*/tests/*"]
+omit = [
+    "*/tests/*",
+    "src/dashboard/routes/mobile_test.py",
+]
+
+[tool.coverage.report]
+show_missing = true
+skip_empty = true
+precision = 1
+exclude_lines = [
+    "pragma: no cover",
+    "if __name__ == .__main__.",
+    "if TYPE_CHECKING:",
+    "raise NotImplementedError",
+    "@abstractmethod",
+]
+# Fail CI if coverage drops below this threshold
+fail_under = 60
+
+[tool.coverage.html]
+directory = "htmlcov"
+
+[tool.coverage.xml]
+output = "coverage.xml"