docs: add MIT LICENSE file

Add LICENSE file with MIT License text to repository root.
README.md already references it; the file was missing.

Includes TDD-verified tests ensuring LICENSE exists and contains
MIT License text, and README references it correctly.

Bump version → 1.15.1

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

compose.yaml

@@ -49,7 +49,7 @@ services:
         crond -f
 
   bot:
-    image: git.codeanddice.ru/toutsu/gmrelay-bot:1.15.0
+    image: git.codeanddice.ru/toutsu/gmrelay-bot:1.15.1
     restart: always
     depends_on:
       db:
@@ -62,7 +62,7 @@ services:
       - gmrelay
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:1.15.0
+    image: git.codeanddice.ru/toutsu/gmrelay-web:1.15.1
     restart: always
     depends_on:
       db: