feat(2024): add Classes collection

[t11z]

What does this do?

Adds the 2024 Classes collection — 12 SRD 5.2.1 class documents, a Zod schema, and tests.

• src/2024/5e-SRD-Classes.json — All 12 core classes: Barbarian, Bard, Cleric, Druid, Fighter, Monk, Paladin, Ranger, Rogue, Sorcerer, Warlock, Wizard
• src/2024/schemas/5e-SRD-Classes.ts — Zod schema following the same structure as the 2014 schema, adapted for 2024 URL conventions
• src/2024/tests/schemas.test.ts — Classes added to the schema test suite
• src/2024/5e-SRD-Subclasses.json — Replaced nyi placeholder key with proper url in all 12 subclass class references

Known gaps

The 2024 Proficiencies collection currently only covers Background-relevant proficiencies (14 entries). As a result:

• proficiencies arrays are omitted — armor, weapon, and saving-throw proficiencies don't exist in the 2024 collection yet
• proficiency_choices use resource_list format pointing to /api/2024/proficiencies rather than explicit option arrays, since individual skill/instrument proficiency entries are also missing
• Some starting equipment is absent where 2024 equipment equivalents don't exist (explorers-pack, entertainers-pack, spellbook)

These gaps should be addressed in a follow-up PR that expands the Proficiencies collection.

How was it tested?

Locally — all 49 tests pass (vitest run), including the schema validation test and the broken-link tables test.

Is there a Github issue this is resolving?

No

Did you update the docs in the API? Please link an associated PR if applicable.

No

[t11z]

Note: This PR resolves the /api/2024/classes/{class}-nyi references
in #1056 by @G-Ambatte. With both PRs landed, the class-level -nyi
URLs in the Features collection would become live links.

Happy to coordinate on merge order or adjust anything to make the
two PRs work together smoothly.