chore: add CORE-003 roadmap task for mypy type safety

- Remove '|| true' from CI type-check job to enforce strict checking
- Begin type narrowing pattern in units.py with _typed property accessors
- Document all 42 type errors across 15 files in roadmap backlog
- Priority: medium, estimated 4-6 hours to complete

Type errors fall into categories:
- Union types not narrowed after __post_init__ coercion
- float() on object types
- Duplicate method definitions
- Provider interface type mismatches

.forgejo/workflows/ci.yaml

@@ -67,10 +67,5 @@ jobs:
           pip install mypy
           pip install nicegui fastapi uvicorn yfinance polars pandas pydantic pyyaml
           pip list
-      - name: Show working directory
-        run: pwd && ls -la
       - name: Run mypy
-        run: |
-          echo "Running mypy on core modules..."
-          mypy app/core app/models app/strategies app/services --ignore-missing-imports --show-error-codes --show-traceback || true
-          echo "Type check completed (warnings allowed during development)"
+        run: mypy app/core app/models app/strategies app/services --ignore-missing-imports --show-error-codes