feat(rates): record hourly rate history

apps/workspaces/models.py

@@ -173,3 +173,53 @@ class WorkspaceUserRate(BaseModel):
                 "currency": self.currency,
             },
         )
+
+
+class HourlyRateHistory(BaseModel):
+    class Scope(models.TextChoices):
+        WORKSPACE = "workspace", "Workspace"
+        PROJECT = "project", "Project"
+
+    workspace = models.ForeignKey(
+        Workspace,
+        on_delete=models.CASCADE,
+        related_name="hourly_rate_history",
+    )
+    user = models.ForeignKey(
+        User,
+        on_delete=models.CASCADE,
+        related_name="hourly_rate_history",
+    )
+    project = models.ForeignKey(
+        "projects.Project",
+        on_delete=models.CASCADE,
+        related_name="hourly_rate_history",
+        null=True,
+        blank=True,
+    )
+    scope = models.CharField(max_length=16, choices=Scope.choices)
+    hourly_rate = models.DecimalField(max_digits=10, decimal_places=2)
+    currency = models.CharField(max_length=3, default="USD")
+    effective_from = models.DateTimeField()
+
+    class Meta:
+        db_table = "hourly_rate_history"
+        ordering = ("effective_from", "created_at")
+        indexes = [
+            models.Index(fields=["workspace", "user", "effective_from"], name="hrh_ws_user_eff_idx"),
+            models.Index(fields=["project", "user", "effective_from"], name="hrh_project_user_eff_idx"),
+        ]
+
+    def get_additional_data(self):
+        return build_workspace_log_metadata(
+            section=SECTION_RATES,
+            workspace_id=self.workspace_id,
+            target_id=self.id,
+            target_label=self.user.full_name or self.user.mobile,
+            extra={
+                "rate_user_id": str(self.user_id),
+                "project_id": str(self.project_id) if self.project_id else None,
+                "scope": self.scope,
+                "currency": self.currency,
+            },
+        )