feat: use connection pool instead of a single shared connection

This is useful for multitask environments (like the API), to avoid
conflicts

server/app/infrastructure/db.py

@@ -0,0 +1,57 @@
+import os
+import time
+import logging
+import psycopg2
+from typing import Generator, Any
+from psycopg2.pool import ThreadedConnectionPool
+from psycopg2.extensions import connection
+from contextlib import contextmanager
+from functools import lru_cache
+
+from domain.exceptions import DatabaseConnectionError
+
+log = logging.getLogger(__name__)
+
+_MIN_CONN = 1
+_MAX_CONN = 10
+
+DB_URI = os.getenv("DATABASE_URI", "postgresql://simugaz:simugaz@db/simugaz")
+
+
+@lru_cache(maxsize=1)
+def get_pool() -> ThreadedConnectionPool:
+    return _create_pool(DB_URI)
+
+
+def _create_pool(
+    uri: str, retries: int = 10, base_delay: float = 1.0
+) -> ThreadedConnectionPool:
+    for attempt in range(retries):
+        try:
+            pool = ThreadedConnectionPool(_MIN_CONN, _MAX_CONN, uri)
+            log.info(
+                "Pool PostgreSQL initialisé (%d–%d connexions)", _MIN_CONN, _MAX_CONN
+            )
+            return pool
+        except psycopg2.OperationalError as e:
+            delay = min(base_delay * 2**attempt, 30.0)
+            log.warning(
+                "Attente PostgreSQL (tentative %d/%d) : %s", attempt + 1, retries, e
+            )
+            time.sleep(delay)
+    raise DatabaseConnectionError(
+        f"Impossible de se connecter après {retries} tentatives"
+    )
+
+
+@contextmanager
+def get_conn() -> Generator[connection, Any, None]:
+    pool = get_pool()
+    conn = pool.getconn()  # type: ignore
+    try:
+        yield conn
+    except Exception:
+        conn.rollback()  # type: ignore
+        raise
+    finally:
+        pool.putconn(conn)  # type: ignore