revert(incidents): remove custom fields from OSS

[singret]

Summary

• Reverts OPE-109 (090b241) — custom field definitions are a Pro-tier feature and belong in regen-pro, not the OSS build
• Resolves conflicts by preserving the OPE-112 additions (ResolvedSince filter, GetBySlackMessageTS) and the OPE-159/160 properties panel redesign
• The custom_fields JSONB column on incidents is retained (it predates OPE-109 — migration 000002) as are all CustomFields model references

What's removed

• custom_field_definitions table, model, and migration (000039)
• Custom field CRUD API handlers and routes
• CustomFieldRepository
• Settings → Incidents page
• Custom fields section in PropertiesPanel, CreateIncidentModal, and IncidentsListPage filters
• frontend/src/api/customFields.ts

What's preserved

• incidents.custom_fields JSONB column and model field (pre-existing)
• OPE-112: ResolvedSince filter and GetBySlackMessageTS on IncidentRepository
• OPE-159/160: full PropertiesPanel redesign (collapsible sections, commander search, HandoffDigest)

Test plan

• Backend compiles without errors
• Settings page no longer has an Incidents nav entry
• Incident detail properties panel renders without custom fields section
• Incident list has no custom field filter controls
• CI passes