fix: replace bare except with typed exceptions in idea/service.py score parsing

[fayevans]

Three bare except: clauses in backend/app/modules/idea/service.py (lines 6507, 6513, 6519) catch BaseException — including KeyboardInterrupt and SystemExit — when all they need to catch is the ValueError from float("") or IndexError from an empty filter result.

Changes:

• Replaced all three except: with except (ValueError, IndexError): in the novelty, feasibility, and impact score parsing blocks
• The fallback to 7.0 still works for malformed input lines
• KeyboardInterrupt and SystemExit now propagate correctly

Added backend/tests/test_pr_09_idea_score_except.py with 7 tests covering normal digits, decimals, empty strings, clamping bounds, and structural verification that KeyboardInterrupt is not caught.