feat: user notification preferences UI and API endpoints

GET/PUT /api/user/notification-preferences with partial update support.
Toggle section in /account Admin tab (Email, SMS, Browser push).
3 new tests: defaults, full update, partial update.
Fix pre-existing timezone bug in exceptional hours tests (Truncate vs time.Date).
Update README, Technical Manual, and gap backlog (#15 struck out).

init-scripts/init-script.sql

@@ -538,13 +538,10 @@ CREATE TABLE admin_notifications (
 CREATE TABLE user_notification_preferences (
     id SERIAL PRIMARY KEY,
     user_id CHAR(12) REFERENCES users(id) ON DELETE CASCADE,
-    notification_type VARCHAR(50) NOT NULL,
-    email_enabled BOOLEAN DEFAULT true,
-    sms_enabled BOOLEAN DEFAULT true,
-    push_enabled BOOLEAN DEFAULT true,
-    created_at TIMESTAMPTZ DEFAULT NOW(),
-    updated_at TIMESTAMPTZ DEFAULT NOW(),
-    UNIQUE(user_id, notification_type)
+    email_enabled BOOLEAN DEFAULT false,
+    sms_enabled BOOLEAN DEFAULT false,
+    browser_push_enabled BOOLEAN DEFAULT false,
+    updated_at TIMESTAMPTZ DEFAULT NOW()
 );
 
 CREATE INDEX idx_user_notification_preferences_user_id ON user_notification_preferences(user_id);
@@ -776,14 +773,12 @@ BEGIN
         'notification_preferences', (
             SELECT COALESCE(json_agg(
                 json_build_object(
-                    'notification_type', unp.notification_type,
-                    'email_enabled',     unp.email_enabled,
-                    'sms_enabled',       unp.sms_enabled,
-                    'push_enabled',      unp.push_enabled,
-                    'created_at',        unp.created_at,
-                    'updated_at',        unp.updated_at
+                    'email_enabled',        unp.email_enabled,
+                    'sms_enabled',          unp.sms_enabled,
+                    'browser_push_enabled', unp.browser_push_enabled,
+                    'updated_at',           unp.updated_at
                 )
-            ORDER BY unp.notification_type), '[]'::json)
+            ), '[]'::json)
             FROM user_notification_preferences unp
             WHERE unp.user_id = target_user_id
         ),