Adds per-bank configurable safety settings for Gemini/Vertex AI:
- New `HINDSIGHT_API_LLM_GEMINI_SAFETY_SETTINGS` env var (JSON array)
- Hierarchical config field so banks can override via Config API
- ContextVar pattern for zero-signature-change per-request override
- All 6 thresholds supported: UNSPECIFIED, OFF, BLOCK_NONE, BLOCK_LOW_AND_ABOVE, BLOCK_MEDIUM_AND_ABOVE, BLOCK_ONLY_HIGH
- UI: Models > Gemini/Vertex AI section with per-category threshold selectors and link to Google docs
- Graceful handling when bank_config_api feature is disabled
- 12 new tests covering config parsing, GeminiLLM behaviour, and context var override
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