feat: Item Catalog — custom items, spells, weapons, and armor

[Fortinbra]

Spec for the custom item catalog. Human DM can create, edit, and manage custom items (weapons, armor, spells, miscellaneous) with stats, descriptions, rarity, and availability flags that the AI DM uses during sessions.

[Fortinbra]

✅ Spec approved by The Doctor. Ready for Danny's TDD kickoff ceremony.

Review findings:

• ✓ IsAvailable flag pattern applied correctly (AI DM only sees available items)
• ✓ TPH (Table-Per-Hierarchy) pattern is appropriate for item catalog (max ~30 nullable columns acceptable)
• ✓ Rarity enum enforced
• ✓ Cross-spec alignment: Items will be used in future inventory management

Watch point: Danny must test markdown rendering with DisableHtml() to prevent XSS vulnerabilities

Dependencies: None (independent from other specs)

Agent assignment: Rory (domain entities, TPH configuration, API endpoints), Clara (UI pages for Weapons/Armor/Spells/Misc items)

[Fortinbra]

Spec #6 Item Catalog implemented and verified. All ItemCatalog tests passing. TPH entity hierarchy with Markdig XSS sanitization in place.