chore: remove unused config fields from v1.13.0 pydantic migration

[dimoschi]

Problem

The Pydantic v2 migration (v1.13.0) and follow-on PRs left several config fields
declared in src/core/config.py that are never read anywhere else in the
codebase.

Dead fields

RolesSettings:

• RANK_FIVE, RANK_TWENTY_FIVE, RANK_FIFTY, RANK_HUNDRED — not referenced
  in RoleManager._ENV_FALLBACK_MAP or scripts/seed_dynamic_roles.py::SEED_DATA
• ACADEMY_CBBH — the CBBH→CWES rename is already handled by
  _CERT_ALIASES = {"CBBH": "CWES"} in role_manager.py; this field itself is
  never read
• APRIL_ROLE_1, APRIL_ROLE_2 — unused

Global:

• APRIL_FLAG_1, APRIL_FLAG_2 — unused
• SLACK_WEBHOOK — only SLACK_FEEDBACK_WEBHOOK is read (src/cmds/core/other.py)
• JIRA_SPOILER_WEBHOOK — only JIRA_WEBHOOK is read (src/cmds/core/other.py)

Proposed fix

Remove these fields. If any were meant to back an in-progress feature (April
Fools event roles, spoiler Jira routing), re-add them in the PR that actually
wires them up.

Notes

Confirm removal doesn't trip extra="forbid" if stray env vars with these
names are still set in Vault/dotenv files, then clean up the corresponding
Vault keys.