docs: clarify safety filter and privacy claims

[katsugtgz]

Concise audit follow-up.

Evidence:

• README said no content filter / prompts never persisted.
• lib/safety.ts defines a CSAM keyword filter.
• lib/orchestrator/orchestrator.ts scans prompts before dispatch.
• lib/db.ts persists job metadata with user/job IDs.

Change:

• Clarifies no model-level refusal layer, but server-side CSAM filter exists.
• Clarifies prompts/images are not stored in app DB, while logs/job metadata may include identifiers.