feat: strengthen module contracts and shared WebUI runtime

src/govoplan_core/db/session.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from collections.abc import Generator, Mapping
+import os
 from typing import Any
 
 from sqlalchemy import create_engine
@@ -10,6 +11,14 @@ from sqlalchemy.orm import Session, sessionmaker
 from govoplan_core.db.query_metrics import instrument_engine
 
 
+_POOL_SETTING_BOUNDS: dict[str, tuple[int, int]] = {
+    "GOVOPLAN_DB_POOL_SIZE": (1, 100),
+    "GOVOPLAN_DB_MAX_OVERFLOW": (0, 200),
+    "GOVOPLAN_DB_POOL_TIMEOUT_SECONDS": (1, 300),
+    "GOVOPLAN_DB_POOL_RECYCLE_SECONDS": (0, 86_400),
+}
+
+
 def default_connect_args(database_url: str) -> dict[str, Any]:
     return {"check_same_thread": False} if database_url.startswith("sqlite") else {}
 
@@ -24,9 +33,42 @@ def create_database_engine(
     merged_connect_args = dict(default_connect_args(database_url))
     if connect_args:
         merged_connect_args.update(connect_args)
+    if not database_url.startswith("sqlite"):
+        kwargs.setdefault(
+            "pool_size",
+            _pool_setting("GOVOPLAN_DB_POOL_SIZE", 5),
+        )
+        kwargs.setdefault(
+            "max_overflow",
+            _pool_setting("GOVOPLAN_DB_MAX_OVERFLOW", 10),
+        )
+        kwargs.setdefault(
+            "pool_timeout",
+            _pool_setting("GOVOPLAN_DB_POOL_TIMEOUT_SECONDS", 30),
+        )
+        kwargs.setdefault(
+            "pool_recycle",
+            _pool_setting("GOVOPLAN_DB_POOL_RECYCLE_SECONDS", 1800),
+        )
     return instrument_engine(create_engine(database_url, pool_pre_ping=pool_pre_ping, connect_args=merged_connect_args, **kwargs))
 
 
+def _pool_setting(name: str, default: int) -> int:
+    raw = os.environ.get(name)
+    if raw is None:
+        return default
+    try:
+        value = int(raw)
+    except ValueError as exc:
+        raise ValueError(f"{name} must be an integer") from exc
+    minimum, maximum = _POOL_SETTING_BOUNDS[name]
+    if not minimum <= value <= maximum:
+        raise ValueError(
+            f"{name} must be between {minimum} and {maximum}",
+        )
+    return value
+
+
 class DatabaseHandle:
     def __init__(self, database_url: str, *, engine: Engine | None = None) -> None:
         self.database_url = database_url