i18n: sweep all remaining error sites onto the code envelope

[sliptonic]

What to build

Mechanical migration of the remaining ~165 HTTPException(detail=...) sites onto the error envelope proven in #55, plus documenting the pydantic 422 validation-error shape as contract (per the #54 decision).

Scope notes:

• Every router and helper that raises client-visible errors adopts the shared helper.
• English detail text is preserved verbatim where reasonable — this sweep adds codes, it does not reword prose.
• Tests that assert on English message text move to asserting codes.

Acceptance criteria

• No raw HTTPException(detail=...) without a code remains in client-visible paths
• 422 validation body shape documented as contract
• Test suite asserts codes rather than English prose for error paths
• Full test suite green with no client-visible behavior change beyond the added fields

Blocked by

• Blocked by i18n tracer: error envelope with code + params on one router #55