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

@@ -9,3 +9,6 @@
 __description__ = "A Python wrapper around the Substack API"
 
 from .api import Api
+from .models import Byline, PostMetadata, ScheduledRelease
+
+__all__ = ["Api", "Byline", "PostMetadata", "ScheduledRelease"]