fix(synthese): order by descending date_min and id_synthese

For Synthese queries involving blurring:
- Add first order_by on descending date_min > this allows to retrieve up-to-50000 more-recently-dated observations from the up-to-100000 observations of the 2 unioned queries
- Set (now) second order_by on id_synthese from ascending to descending > this allows, for several observations with same date_min, to prioritize more-recently-added-in-database observations

Notes:
- We want to preserve the final order_by on `allowed_geom_cte.c.priority` and the distinct clause on id_synthese, to keep only precise geom for obs retrieved in both precise and blurred subqueries: this requires to:
  - Keep the order_by on id_synthese
- The wish to retrieve "more recent" observations may be understood in two ways:
  - Meant as "more-recently-dated": operated by the first order_by > it is thus assumed this is the preferred meaning given to "more recent"
  - Meant as "more-recently-added-in-database": operated by the second order_by > it is thus assumed that for two observations with the same date_min we qualify as "more recent" the one with the greater id_synthese

Solves #3249

Co-authored-by: VincentCauchois <vincent.cauchois@mnhn.fr>
Co-authored-by: Christophe Ramet <christophe.ramet@mnhn.fr>

backend/geonature/core/gn_synthese/utils/blurring.py

@@ -62,7 +62,7 @@ def build_blurred_precise_geom_queries(
         columns.append(sa.literal(0).label("size_hierarchy"))
     precise_geom_query = SyntheseQuery(
         Synthese,
-        sa.select(*columns).where(sa.and_(*where_clauses)).order_by(Synthese.id_synthese.desc()),
+        sa.select(*columns).where(sa.and_(*where_clauses)).order_by(Synthese.date_min.desc()),
         filters=dict(filters),  # not to edit the actual filter object
     )
 
@@ -100,7 +100,7 @@ def build_blurred_precise_geom_queries(
             == Synthese.id_nomenclature_sensitivity
         )
         .where(sa.and_(*where_clauses))
-        .order_by(Synthese.id_synthese.desc()),
+        .order_by(Synthese.date_min.desc()),
         filters=dict(filters),
         query_joins=sa.join(
             Synthese,
@@ -182,8 +182,12 @@ def build_synthese_obs_query(observations, allowed_geom_cte, limit):
                 allowed_geom_cte, allowed_geom_cte.c.id_synthese == VSyntheseForWebApp.id_synthese
             )
         )
-        .order_by(VSyntheseForWebApp.id_synthese, allowed_geom_cte.c.priority)
-        .distinct(VSyntheseForWebApp.id_synthese)
+        .order_by(
+            VSyntheseForWebApp.date_min.desc(),
+            VSyntheseForWebApp.id_synthese.desc(),
+            allowed_geom_cte.c.priority,
+        )
+        .distinct(VSyntheseForWebApp.date_min, VSyntheseForWebApp.id_synthese)
         .limit(limit)
     )
     return obs_query