Add first-class Trailer data models

[roncodes]

Summary

• add Trailer, towing connection, attachable Trailer, and maintenance-subject models
• add Trailer and connection serializers plus polymorphic Device and Equipment support
• expose Vehicle relationships for current Trailer operations
• cover Trailer discriminator, relationships, and polymorphic normalization/serialization

Validation

• pnpm lint
• pnpm build
• Trailer-focused browser assertions execute successfully; the local Testem process did not terminate cleanly after Chrome completed, so CI remains the authoritative full browser gate

Related PRs

• Fleet-Ops: Add first-class Trailer operations fleetops#309
• Postman: Add Trailer API lifecycle collection postman#58
• fleetbase.io: Document first-class Trailer operations fleetbase.io#96