Add credential count cap per subject to prevent storage abuse

[michaelvic123]

Description

A single subject can accumulate unlimited credentials, causing unbounded storage growth. Add a configurable cap on credentials per subject.

Tasks

• Add max_credentials_per_subject: u32 to contract config (set on initialize, default 100)
• Check cap in issue_credential before creating new credential
• Panic with clear message if cap exceeded
• Add admin function update_credential_cap(env, admin, new_cap)
• Add tests: issue up to cap, exceed cap panics, admin updates cap

Acceptance Criteria

• Cap enforced on issuance
• Admin can update cap
• Tests pass

Complexity: Medium (150 pts)