fix: actually reject zero-amount goal contributions (#4)

`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>

backend/app/schemas/goal.py

@@ -1,7 +1,7 @@
 from datetime import date as date_
 from decimal import Decimal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 
 
 class GoalCreate(BaseModel):
@@ -19,10 +19,20 @@ class GoalUpdate(BaseModel):
 class GoalContributionCreate(BaseModel):
     # Negative allowed — a withdrawal from the goal is still a contribution
     # to its running total, just in the other direction. Zero is pointless.
-    amount: Decimal = Field(ne=0)
+    amount: Decimal
     date: date_
     note: str | None = Field(default=None, max_length=200)
 
+    @field_validator("amount")
+    @classmethod
+    def _reject_zero(cls, value: Decimal) -> Decimal:
+        # Has to be a validator: pydantic has no "not equal" constraint, so
+        # the Field(ne=0) this replaces was parsed as an unknown keyword,
+        # stored as schema metadata, and never actually checked anything.
+        if value == 0:
+            raise ValueError("amount must not be zero")
+        return value
+
 
 class GoalRead(BaseModel):
     id: int