feat(desktop-notifications): add new table for session management

This feature introduces a new session table which is needed by the `icinga-notifications-web` project. It stores the current authenticated sessions into the table which allows the background daemon (php-based) to validate a connection and its corresponding browser to an authenticated session.
Read #118 for more information about this addition.

schema/pgsql/schema.sql

@@ -277,3 +277,32 @@ CREATE TABLE incident_history (
     CONSTRAINT pk_incident_history PRIMARY KEY (id),
     FOREIGN KEY (incident_id, rule_escalation_id) REFERENCES incident_rule_escalation_state(incident_id, rule_escalation_id)
 );
+
+CREATE OR REPLACE FUNCTION now_extended() RETURNS bigint
+    LANGUAGE SQL AS
+$$
+SELECT ((EXTRACT(EPOCH FROM CURRENT_TIMESTAMP) * 1_000) :: bigint);
+$$;
+
+CREATE TABLE session
+(
+    id               VARCHAR(256) NOT NULL,
+    username         VARCHAR(254) NOT NULL,
+    device_id        VARCHAR(8)   NOT NULL,
+    authenticated_at bigint       NOT NULL DEFAULT now_extended(),
+
+    CONSTRAINT pk_id_username_device PRIMARY KEY (id, username, device_id)
+);
+
+CREATE INDEX idx_id ON session (
+                                id
+    );
+
+CREATE INDEX idx_authenticated ON session (
+                                           authenticated_at
+    );
+
+CREATE INDEX idx_username_device ON session (
+                                             username,
+                                             device_id
+    );