Modernized codebase with tooling configuration and CI/CD workflows.

- Replaced legacy typing (Optional, List, Type, Union, Tuple) with PEP 604/585 equivalents.
- Added pyproject.toml with configurations for hatch-vcs, mypy, ruff, and codespell.
- Added CI workflows for formatting, linting, type checking, and spelling.
- Added CD workflow for building and uploading plugin artifacts on push to master and version tags.
- Added dependency auditing workflow with pip-audit.
- Added comprehensive docstrings and inline comments across all modules.
- Fixed User-Agent header using hardcoded version instead of actual plugin version.
- Fixed grammar and terminology in log messages and comments.
- Removed unreachable error handling branch in unsubscribe command.

LICENSE.txt

@@ -1,6 +1,7 @@
+
                                  Apache License
                            Version 2.0, January 2004
-                        https://www.apache.org/licenses/
+                        http://www.apache.org/licenses/
 
    TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
 
@@ -174,3 +175,28 @@
       of your accepting any such warranty or additional liability.
 
    END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
+      boilerplate notice, with the fields enclosed by brackets "[]"
+      replaced with your own identifying information. (Don't include
+      the brackets!)  The text should be enclosed in the appropriate
+      comment syntax for the file format. We also recommend that a
+      file or class name and description of purpose be included on the
+      same "printed page" as the copyright notice for easier
+      identification within third-party archives.
+
+   Copyright [yyyy] [name of copyright owner]
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.