🔧(backend) enable psycopg-pool allowing configuring min and max size

We enable the pool option on the DB configuration. We want to allow the
configuration of the min and max sixe in a first time. They can be
configured using the settings DB_PSYCOPG_POOL_MIN_SIZE and
DB_PSYCOPG_POOL_MAX_SIZE. They have their default value to 4 and None.

docs/env.md

@@ -46,6 +46,8 @@ These are the environment variables you can set for the `impress-backend` contai
 | DB_NAME                                         | Name of the database                                                                                                                                                       | impress                                                                 |
 | DB_PASSWORD                                     | Password to authenticate with                                                                                                                                              | pass                                                                    |
 | DB_PORT                                         | Port of the database                                                                                                                                                       | 5432                                                                    |
+| DB_PSYCOPG_POOL_MIN_SIZE                        | The psycopg min pool size                                                                                                                                                  | 4                                                                       |
+| DB_PSYCOPG_POOL_MAX_SIZE                        | The psycopg max pool size                                                                                                                                                  | None                                                                    |
 | DB_USER                                         | User to authenticate with                                                                                                                                                  | dinum                                                                   |
 | DJANGO_ALLOWED_HOSTS                            | Allowed hosts                                                                                                                                                              | []                                                                      |
 | DJANGO_CELERY_BROKER_TRANSPORT_OPTIONS          | Celery broker transport options                                                                                                                                            | {}                                                                      |

src/backend/impress/settings.py

@@ -99,6 +99,19 @@ class Base(Configuration):
                 "localhost", environ_name="DB_HOST", environ_prefix=None
             ),
             "PORT": values.Value(5432, environ_name="DB_PORT", environ_prefix=None),
+            "OPTIONS": {
+                # https://www.psycopg.org/psycopg3/docs/api/pool.html#psycopg_pool.ConnectionPool
+                "pool": {
+                    "min_size": values.IntegerValue(
+                        4, environ_name="DB_PSYCOPG_POOL_MIN_SIZE", environ_prefix=None
+                    ),
+                    "max_size": values.IntegerValue(
+                        None,
+                        environ_name="DB_PSYCOPG_POOL_MAX_SIZE",
+                        environ_prefix=None,
+                    ),
+                }
+            },
         }
     }
     DEFAULT_AUTO_FIELD = "django.db.models.AutoField"