TICKET-002: Targets CRUD with single-active-target invariant

- POST auto-closes previous target in one transaction
- GET /targets (ordered), GET /targets/current (404 when none)
- PUT with invariant enforcement (409 on double-active attempt)
- Service-layer historical target lookup for TICKET-004
- Full suite green (62 passed)

backend/schemas.py

@@ -117,6 +117,36 @@ class LogEntryRead(BaseModel):
 # ── Targets ──────────────────────────────────────────────────────────────────
 
 
+class TargetCreate(BaseModel):
+    """Schema for POST /api/targets. end_date is auto-managed by the service."""
+
+    start_date: date  # client-supplied YYYY-MM-DD (spec §8.1 rule 8)
+    calories: int = Field(gt=0)
+    protein_g: float | None = Field(default=None, ge=0)
+    carbs_g: float | None = Field(default=None, ge=0)
+    fat_g: float | None = Field(default=None, ge=0)
+
+
+class TargetUpdate(BaseModel):
+    """Schema for PUT /api/targets/{id}. All fields optional — only supplied
+    fields are updated. Setting end_date to null will reactivate a historical
+    target only if no other active target exists."""
+
+    start_date: date | None = None
+    end_date: date | None = None
+    calories: int | None = Field(default=None, gt=0)
+    protein_g: float | None = Field(default=None, ge=0)
+    carbs_g: float | None = Field(default=None, ge=0)
+    fat_g: float | None = Field(default=None, ge=0)
+
+    @model_validator(mode="after")
+    def _check_date_range(self):
+        if self.start_date is not None and self.end_date is not None:
+            if self.end_date <= self.start_date:
+                raise ValueError("end_date must be after start_date")
+        return self
+
+
 class TargetRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)