fix: add encoding="utf-8" to review_storage.py open() calls

[nathaq]

Hey! I noticed that review_storage.py has 9 open() calls that read and write JSON files without specifying encoding="utf-8". On systems where the default locale is not UTF-8 (Windows with cp1252, minimal containers), this will either corrupt non-ASCII data silently or throw UnicodeDecodeError.

The fix adds encoding="utf-8" to every text/JSON open() call in the file -- both reads (get_review, list_reviews, list_improvement_requests, get_improvement_request) and writes (_save_record, create_improvement_request, update_improvement_request).

review_storage.py handles review reports, findings, and improvement request descriptions -- all of which regularly contain non-ASCII text (Chinese, accented characters, etc.), so this is especially important here.

Files changed:

• backend/app/storage/review_storage.py -- added encoding="utf-8" to 9 open() calls
• backend/tests/test_pr_02_review_encoding.py -- round-trip tests with Chinese and accented text

Verified: All 3 tests pass in the sandbox container.