Paginated list of row-level validation errors for a Usage File.

Unions three existing error sources:

• UsageFile.error_detail (file-level — one entry, row=None)
• UsageRecord.error_details/codes (per-record invalid rows)
• ItemCreationErrors (dynamic-item creation failures)

Each entry mapped to {column?, code, message, raw_value?}. The row field is omitted because source-row index isn't persisted today.

Each per-row source is capped at VALIDATION_ERRORS_MAX_PER_SOURCE to bound memory and DB I/O for pathologically invalid files. Beyond that cap callers see a representative sample; the response's truncated flag is set so they know more errors exist.

total is the count of errors collected into this response (what the items are paginated over). When truncated is true the real number of errors in the DB is higher than total; we intentionally do not run a COUNT(*) to surface it (see the cap + 1 fetch below).