api/ui: replace default regression name refund_limit_regression

[CodewithJha]

Problem

Hosted still defaults regression names to the refund demo:

• RegressionSaveRequest.name: str = "refund_limit_regression" in apps/api/src/mutiny_api/schemas.py
• UI default string in apps/web/src/app/campaign/[id]/page.tsx (~refund_limit_regression)

Documented as P3-2 in PRODUCTION_READINESS. Product seeds are policy-derived (M-PR6), but this default re-centers the refund narrative.

Why it matters

Observe-only Hosted UX should not imply every finding is a refund exploit.

Proposed direction

Default to a generic name such as discovered_violation / policy_violation, or derive from first target_rule_ids[0] when present. Update UI placeholder to match.

Acceptance criteria

• API default name is domain-generic (or rule-id-based)
• Campaign UI placeholder/default matches
• Existing tests that hardcode the old name updated
• No schema break beyond default string

Testing expectations

Relevant API/web tests / grep for old string cleaned up.

Difficulty

XS · good first issue