ci(coverage): raise threshold to 75% and refine omit patterns

- Omit routes.py and __init__.py (HTTP layer, tested by integration tests)
- Omit preparator_v4.py and llm_provider.py (external dependencies)
- Unit test coverage now measures business logic only: 85%+
- Raise fail_under from 60% to 75%

Author: VTvito

pyproject.toml

@@ -55,8 +55,15 @@ select = ["E", "F", "W", "I"]
 
 [tool.coverage.run]
 source = ["services", "preparator", "ai_agent"]
-omit = ["tests/*", "*/run.py"]
+omit = [
+    "tests/*",
+    "*/run.py",
+    "services/*/app/routes.py",
+    "services/*/app/__init__.py",
+    "preparator/preparator_v4.py",
+    "ai_agent/llm_provider.py",
+]
 
 [tool.coverage.report]
 show_missing = true
-fail_under = 60
+fail_under = 75