feat(raids): Add start_date to raids.json

[HU-Lee]

Summary

• Add start_date (season start, ISO 8601) to every entry of batorment/v3/raids.json.
• The web uses it to label ongoing and upcoming seasons. Every season runs one week from its start date.

Implementation notes

• Additive only. Ordering and existing fields are unchanged.
• Current readers ignore the new field: the web reads it as an optional property, and ba-analyzer decodes raids.json with json.Unmarshal into a struct without it.

Evidence

• Go: Static checks: PASS (scope: whole-repo)
• Dry run of process-raid --dry-run --recent 0 against production data: all 56 entries carry start_date, e.g. S93-0 2026-09-30T04:00:00+09:00 and 3S38-1 2026-10-14T04:00:00+09:00.