feat(crm): Prospecto — medio de contacto preferido + fix visualización de documentos (Fase C)

- Prospecto (lead): se conserva "Origen" y se agrega "Medio de contacto preferido"
  (catálogo medio_contacto). Backend leads.preferred_contact_method + migración
  f0a1b2c3d4e5 reversible.
- Bug documentos: endpoint proxy GET /v1/crm/uploads/download transmite el archivo
  por el backend (valida aislamiento tenant/company) — evita la URL prefirmada al
  host interno minio:9000. RelatedManager.openDoc usa blob→objectURL.

Suite backend en verde (109). svelte-check sin errores nuevos.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/api/v1/modules/crm/leads/dto.py

@@ -11,6 +11,7 @@ class LeadCreate(BaseModel):
     phone: str | None = Field(None, max_length=40)
     company_name: str | None = Field(None, max_length=255)
     source: str | None = Field(None, max_length=60)
+    preferred_contact_method: str | None = Field(None, max_length=20)
     status: str = Field("new", max_length=20)
     estimated_value: Decimal | None = Field(None, ge=0, max_digits=14, decimal_places=2)
     owner_user_id: str | None = Field(None, max_length=64)
@@ -24,6 +25,7 @@ class LeadUpdate(BaseModel):
     phone: str | None = Field(None, max_length=40)
     company_name: str | None = Field(None, max_length=255)
     source: str | None = Field(None, max_length=60)
+    preferred_contact_method: str | None = Field(None, max_length=20)
     status: str | None = Field(None, max_length=20)
     estimated_value: Decimal | None = Field(None, ge=0, max_digits=14, decimal_places=2)
     owner_user_id: str | None = Field(None, max_length=64)
@@ -50,6 +52,7 @@ class LeadResponse(BaseModel):
     phone: str | None
     company_name: str | None
     source: str | None
+    preferred_contact_method: str | None = None
     status: str
     estimated_value: Decimal | None
     owner_user_id: str | None