feat(api): harden the bulk upload CSV contract (#2362)

[jh-RLI]

Summary of the discussion

This PR stacks ontop of #2364 wait until its merged and rebase this PR.

Part of #2362 (Slice 3 — CSV contract hardening). Implements the explicit
CSV contract on top of the bulk upload tracer bullet:

• Header preflight, before the body streams: duplicate column names,
  names not in the table, and missing required columns (NOT NULL without a
  default) are each rejected with a 400 naming the offending columns —
  a wrong column mapping costs milliseconds, not gigabytes.
• UTF-8 BOM stripped from the header, so Excel exports work unmodified.
• Empty field = NULL, always (FORCE_NULL on all uploaded columns),
  whether the field is quoted or not. Deliberate deviation from COPY's
  native CSV rule (quoted "" = empty string): many common writers quote
  every field and would otherwise have their NULLs silently stored as empty
  strings. Do not "fix" this back to Postgres defaults — it would silently
  corrupt NULL handling for clients.
• Sanitized, located errors: failures return the database's data-level
  message plus the CSV line number (header-adjusted) and column — e.g.
  invalid input syntax for type bigint: "boom" (CSV line 3, column id) —
  and never raw SQL, server context dumps, or internal paths. When the
  server provides no diagnostics (connection lost mid-COPY), the message
  stays generic instead of echoing socket paths.

Known minor limitation (accepted): Postgres reports COPY records; a quoted
field containing embedded newlines earlier in the file shifts the reported
line relative to physical file lines.

Tests: module grows to 20 HTTP-seam tests — one per contract rule
(duplicates, missing-required incl. serial-default interplay, BOM, column
order freedom, quoted/unquoted NULL, line+column error shape with
no-internals assertions). Full suite green. Changelog entry included.

Workflow checklist

Automation

Closes #

Part of #2362

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