docs: add panic-as-DoS threat to THREAT_MODEL.md

[ReinaMaze]

Threat Model Update: Panic-as-DoS Risk Documentation

Add comprehensive panic-as-denial-of-service threat coverage to THREAT_MODEL.md.

Changes

• New Section 4: Panic-as-Denial-of-Service (DoS) Risk
  • Explains threat: panic in entry points can be exploited to halt contract
  • Documents affected components with specific line numbers:
    • timelock_manager.rs (lines 49, 52, 60, 74, 77, 85, 268, 300, 325)
    • endorsement_registry.rs (line 26)
    • bookmark_registry.rs (line 28)
    • dependency_registry.rs (lines 100, 111, 122, 130)
    • review_registry.rs (lines 1074, 1075)
    • project_registry.rs (lines 1973, 1974)
    • admin_manager.rs (line 23)
  • Includes code examples for each panic site
  • Provides mitigation strategy: replace panic/unwrap with Result returns, add bounds checking, validate UTF-8
  • Lists related issues for future fixes

close #383