Request/response transformation middleware

Implement as a dedicated transformers/ directory inside the Edge Function with one file per adapter (xledger-transformer.ts, dynamics-transformer.ts, bufdir-transformer.ts) plus a shared types.ts. Use discriminated union types for IntegrationResult to make error handling exhaustive at compile time. For date handling, always convert to UTC before serialization — do not assume server timezone. For Bufdir CSV, read column order dynamically from bufdir_column_schema at runtime rather than hardcoding, since schema versions can change.

Currency: use integer arithmetic (store as øre/cents internally) to avoid floating-point drift, only convert to decimal at serialization boundary. Keep transformers stateless and free of I/O so they can be tested without Supabase dependencies.

Write unit tests in Deno test framework covering: (1) Xledger transformer — valid activity record produces correct JSON structure; (2) Dynamics transformer — expense claim maps OData type annotations correctly; (3) Bufdir CSV transformer — column_mappings from bufdir_column_schema drive column order; (4) Date normalizer — input in Norway timezone (Europe/Oslo) serializes to correct UTC ISO string; (5) Currency precision — 0.005 rounds to 0.01, 0.015 rounds to 0.02 (banker's rounding); (6) TransformationError thrown for missing required field. Integration tests: invoke Edge Function locally with Supabase CLI and assert output payload matches snapshot. Target 90% line coverage on transformer module.