RE2-unsafe regex patterns: audit production segments and decide fallback policy

The translator returns `None` for any `REGEX` condition whose pattern contains:
- Backreferences (`\1`–`\9`)
- Lookarounds (`(?=`, `(?!`, `(?<=`, `(?

Start with `_RE2_UNSAFE` in `src/flagsmith_sql_flag_engine/translator.py` and inspect the engine-test-data suite. Run the proposed `REGEXP_LIKE` audit against the segments table’s `rules` JSON column to measure usage of backreferences and lookarounds. Done means the prevalence is recorded, a fallback policy is chosen, and the corresponding behavior is implemented and validated.

索引モデルが issue の本文から書いたものです。