`GoalContributionCreate.amount` used `Field(ne=0)`, but pydantic has no
"not equal" constraint — the keyword was accepted as unknown schema
metadata and never validated anything. `POST /api/goals/{id}/contributions`
with `"amount": "0"` returned 201 and wrote a contribution that moves the
goal's progress by nothing, and the comment right above the field says zero
is pointless.
Replaced with a field_validator so the rule is enforced, deliberately not
with `gt=0`: negative contributions are intentional (money taken back out of
a goal), and the added tests pin both halves of that.
This also clears the `PydanticDeprecatedSince20: Using extra keyword
arguments on Field (Extra keys: 'ne')` warning that the test suite printed
on every run — that warning was the symptom.
Claude-Session: https://claude.ai/code/session_01UmuMDFrwdvQkce885VaaTh
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
