feat: Upgrade to v2.0 with major download stability overhaul

This commit marks a major upgrade to version 2.0, focusing entirely on resolving the critical stability issues encountered during parallel downloads.

Key changes include:

- **New Stable Download Mechanism:** Replaced the previous download logic with a manual, chunk-based implementation. This introduces configurable delays (`chunk_delay_ms`) between chunks, effectively preventing `GetFileRequest` flood waits and enabling truly stable parallel downloads.

- **Resolved Expired File References:** Workers now fetch a fresh file reference on-demand just before a download begins. This completely resolves the `File reference has expired` error, which previously caused downloads to fail and create 0-byte files.

- **Intelligent State Management:** The state persistence logic has been reworked to track the last successfully *downloaded* file's message ID, rather than the last scanned ID. This ensures no pending downloads are lost if the script is restarted.

- **Comprehensive Code & Config Refactor:** Performed a major cleanup by removing a significant amount of dead code, unused functions, and obsolete settings. The `config.json` structure has also been refactored to be more intuitive, with clear `premium_settings` and `free_settings` blocks.

- **Full Documentation Update:** The `CHANGELOG.md`, `README.md`, and `QUICK_START.md` files have been completely updated to reflect all the new features, the new configuration structure, and the improved stability of version 2.0.

Author: challal

.gitignore

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+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
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+# C extensions
+*.so
+
+# Distribution / packaging
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+develop-eggs/
+dist/
+downloads/
+eggs/
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+lib/
+lib64/
+parts/
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+var/
+wheels/
+pip-wheel-metadata/
+share/python-wheels/
+*.egg-info/
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+*.egg
+MANIFEST
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+# PyInstaller
+*.manifest
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+# Installer logs
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+# Unit test / coverage reports
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+*.cover
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+# Translations
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+# Django stuff:
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+db.sqlite3
+db.sqlite3-journal
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+# Flask stuff:
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+# Scrapy stuff:
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+# Sphinx documentation
+docs/_build/
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+# PyBuilder
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+# Jupyter Notebook
+.ipynb_checkpoints
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+# IPython
+profile_default/
+ipython_config.py
+
+# pyenv
+.python-version
+
+# pipenv
+Pipfile.lock
+
+# PEP 582
+__pypackages__/
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+# Celery stuff
+celerybeat-schedule
+celerybeat.pid
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+# SageMath parsed files
+*.sage.py
+
+# Environments
+.env
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+env/
+venv/
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+env.bak/
+venv.bak/
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+# Spyder project settings
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+# Rope project settings
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+# mkdocs documentation
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+# mypy
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+# Pyre type checker
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+# Telegram session files
+*.session
+*.session-journal
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+# Project specific
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+*.log
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+# IDE files
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+*.swp
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+*~
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+# OS files
+.DS_Store
+.DS_Store?
+._*
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+Thumbs.db
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+# Node modules (if any)
+node_modules/
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+# Temporary files
+*.tmp
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+# Backup files
+*.bak
+*.backup
+
+# Assets (but keep logo.svg)
+assets/*
+!assets/logo.svg