feat: add models module, API helpers, and Post refactoring

- Add models.py with Byline, PostMetadata, ScheduledRelease dataclasses
- Add schedule_release(), get_post_metadata(), make_post_free() to Api
- Extract _authenticate() and _resolve_publication() from __init__
- Add _get/_post/_put/_delete HTTP helpers, refactor all methods to use them
- Add _handle_response allow_empty param with proper error chaining
- Add _PRODUCTION_SUBDOMAINS safety guard
- publish_draft now auto-runs prepublish validation
- Use pathlib.Path instead of os.path/open for file operations
- Post: dispatch table _ADD_HANDLERS replaces if/elif chain
- Post: add paywall() and add_subscribe_button() methods
- Post: extract _parse_markdown_blocks, _upload_or_passthrough, _render_image
- Post: code_block uses match/case, _process_line handles inline headings
- Refactored from_markdown() with HR support and better line merging
- MCP: remove separate prepublish_draft tool (now automatic)
- Add type annotations throughout all modules

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: svaningelgem

substack/__init__.py

@@ -1,11 +1,14 @@
-"""A library that provides a Python interface to the Substack API."""
-
-__author__ = "Paolo Mazza"
-__email__ = "mazzapaolo2019@gmail.com"
-__license__ = "MIT License"
-__version__ = "0.1.21"
-__url__ = "https://github.com/ma2za/python-substack"
-__download_url__ = "https://pypi.python.org/pypi/python-substack"
-__description__ = "A Python wrapper around the Substack API"
-
-from .api import Api
+"""A library that provides a Python interface to the Substack API."""
+
+__author__ = "Paolo Mazza"
+__email__ = "mazzapaolo2019@gmail.com"
+__license__ = "MIT License"
+__version__ = "0.1.21"
+__url__ = "https://github.com/ma2za/python-substack"
+__download_url__ = "https://pypi.python.org/pypi/python-substack"
+__description__ = "A Python wrapper around the Substack API"
+
+from .api import Api
+from .models import Byline, PostMetadata, ScheduledRelease
+
+__all__ = ["Api", "Byline", "PostMetadata", "ScheduledRelease"]