Made database connection pool size configurable. Fixes #11

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/config.rs

@@ -43,11 +43,18 @@ pub struct Database {
     password: String,
     db_name: String,
     host: String,
-    port: Option<u16>
+    port: Option<u16>,
+    max_connections: Option<u32>
 }
 
 #[cfg(feature = "ssr")]
 impl Database {
+    /// Upper limit of open database connections, 10 by default. With many instances on one
+    /// PostgreSQL server the sum of the limits must fit into its `max_connections`.
+    pub fn max_connections(&self) -> u32 {
+        self.max_connections.unwrap_or(10).max(1)
+    }
+
     pub fn con_string(&self) -> String {
         format!("postgres://{}:{}@{}:{}/{}", self.user, self.password, self.host, self.port.unwrap_or(5432), self.db_name)
     }
@@ -358,6 +365,32 @@ mod tests {
         assert_eq!(database.con_string(), "postgres://u:p@db.example.com:6543/db");
     }
 
+    #[test]
+    fn max_connections_defaults_to_ten() {
+        let database: Database = toml::from_str(
+            "user = 'u'\npassword = 'p'\ndb_name = 'db'\nhost = 'localhost'").unwrap();
+
+        assert_eq!(database.max_connections(), 10);
+        // The sample config has the key commented out
+        assert_eq!(sample().database().max_connections(), 10);
+    }
+
+    #[test]
+    fn max_connections_can_be_configured() {
+        let database: Database = toml::from_str(
+            "user = 'u'\npassword = 'p'\ndb_name = 'db'\nhost = 'localhost'\nmax_connections = 3").unwrap();
+
+        assert_eq!(database.max_connections(), 3);
+    }
+
+    #[test]
+    fn max_connections_is_at_least_one() {
+        let database: Database = toml::from_str(
+            "user = 'u'\npassword = 'p'\ndb_name = 'db'\nhost = 'localhost'\nmax_connections = 0").unwrap();
+
+        assert_eq!(database.max_connections(), 1);
+    }
+
     #[test]
     fn smtp_mailing_config() {
         let mailing: Mailing = toml::from_str(