Replace magic values with named constants and unify yt-dlp H.264 format selection

[Copilot]

This PR removes several unexplained literals in GitHub integration/retry paths and standardizes yt-dlp format selection so preview and main download flows use the same H.264-first strategy. It also aligns one type annotation with broader Python compatibility guidance.

• gh.py: replace magic numbers with named constants

  • Introduces:
    • MIN_GITHUB_TOKEN_LENGTH = 30 (with rationale comment)
    • GITHUB_COMMIT_RETRY_DELAY_FACTOR = 0.6
    • VIDEOS_JSON_RETRY_DELAY_FACTOR = 0.4
    • ENTRY_VERIFICATION_DELAY_FACTOR = 2.0
  • Replaces inline usages in token validation and retry/backoff calculations.

• gh.py: typing compatibility cleanup

  • Updates _load_videos_json_and_sha return annotation from tuple[...] to Tuple[...] using typing.Tuple.

• tg.py: consolidate and standardize yt-dlp format selector

  • Adds module-level constant:
    • YT_DLP_H264_PREFERRED_FORMAT
  • Replaces duplicated inline selectors in download dry-run and actual download commands.
  • Uses documented codec negation syntax (vcodec!=av01) instead of vcodec!*=av01.

• tg.py: align preview download behavior with main flow

  • Updates /get_video_preview download command to use the same shared H.264-preferring selector, preventing preview/main codec-selection drift.

# tg.py
YT_DLP_H264_PREFERRED_FORMAT = (
    "bestvideo[ext=mp4][vcodec^=avc]+bestaudio[ext=m4a]/"
    "bestvideo[ext=mp4][vcodec!=av01]+bestaudio[ext=m4a]/"
    "bestvideo[vcodec^=avc]+bestaudio/best"
)

# reused in all yt-dlp download paths
"-f", YT_DLP_H264_PREFERRED_FORMAT

Original prompt

Please apply the following diffs and create a pull request.
Once the PR is ready, give it a title based on the messages of the fixes being applied.

[{"message":"The magic number 30 for token length validation is unexplained. Consider defining this as a named constant (e.g., `MIN_GITHUB_TOKEN_LENGTH = 30`) with a comment explaining the rationale, or remove this heuristic check if token format validation isn't critical.","fixFiles":[{"filePath":"gh.py","diff":"diff --git a/gh.py b/gh.py\n--- a/gh.py\n+++ b/gh.py\n@@ -18,6 +18,10 @@\n \n # GitHub API imports\n from github import Github, InputGitTreeElement\n+\n+# Heuristic minimum length check to catch obviously invalid/placeholder tokens.\n+# This is not a strict format validator for all possible GitHub token types.\n+MIN_GITHUB_TOKEN_LENGTH = 30\n from github.GithubException import GithubException\n \n # --- Logging Setup ---\n@@ -289,7 +293,7 @@\n \n def authenticate_github():\n     \"\"\"Authenticates with GitHub API using a Personal Access Token.\"\"\"\n-    if not GITHUB_ACCESS_TOKEN or len(GITHUB_ACCESS_TOKEN) < 30: # Basic check for placeholder/empty token\n+    if not GITHUB_ACCESS_TOKEN or len(GITHUB_ACCESS_TOKEN) < MIN_GITHUB_TOKEN_LENGTH: # Basic check for placeholder/empty token\n         logger.error(\"GITHUB_ACCESS_TOKEN is not set or is invalid. Cannot authenticate with GitHub.\")\n         return None\n     try:\n"}]},{"message":"The magic number 0.6 for delay calculation is unexplained. Consider defining this as a named constant (e.g., `GITHUB_COMMIT_RETRY_DELAY_FACTOR = 0.6`) to clarify the retry backoff strategy.","fixFiles":[{"filePath":"gh.py","diff":"diff --git a/gh.py b/gh.py\n--- a/gh.py\n+++ b/gh.py\n@@ -8,6 +8,8 @@\n from typing import Tuple, Optional, Callable\n from env_config import get_env\n \n+GITHUB_COMMIT_RETRY_DELAY_FACTOR = 0.6\n+\n # Google Drive API imports\n from google.oauth2.credentials import Credentials\n from google_auth_oauthlib.flow import InstalledAppFlow\n@@ -348,7 +350,7 @@\n             status = getattr(e, \"status\", None)\n             is_conflict = status == 409 or \"does not match\" in str(e).lower()\n             if is_conflict and attempt < max_retries:\n-                delay = 0.6 * attempt\n+                delay = GITHUB_COMMIT_RETRY_DELAY_FACTOR * attempt\n                 logger.warning(\n                     f\"GitHub conflict while committing '{file_path}' (attempt {attempt}/{max_retries}). \"\n                     f\"Retrying in {delay:.1f}s...\"\n"}]},{"message":"The return type annotation uses lowercase `tuple` which requires Python 3.9+. For broader compatibility, consider using `Tuple` from the typing module (already imported on line 8) or add a comment documenting the minimum Python version requirement.","fixFiles":[{"filePath":"gh.py","diff":"diff --git a/gh.py b/gh.py\n--- a/gh.py\n+++ b/gh.py\n@@ -365,7 +365,7 @@\n     return None\n \n \n-def _load_videos_json_and_sha(repo, branch: str) -> tuple[list, Optional[str]]:\n+def _load_videos_json_and_sha(repo, branch: str) -> Tuple[list, Optional[str]]:\n     \"\"\"Loads videos.json list and current blob SHA (if file exists).\"\"\"\n     try:\n         contents = repo.get_contents(GITHUB_VIDEOS_JSON_PATH, ref=branch)\n"}]},{"message":"The magic number 0.4 for delay calculation is unexplained. Consider defining this as a named constant (e.g., `VIDEOS_JSON_RETRY_DELAY_FACTOR = 0.4`) to clarify the retry backoff strategy and distinguish it from other retry delays in the codebase.","fixFiles":[{"filePath":"gh.py","diff":"diff --git a/gh.py b/gh.py\n--- a/gh.py\n+++ b/gh.py\n@@ -8,6 +8,8 @@\n from typing import Tuple, Optional, Callable\n from env_config import get_env\n \n+VIDEOS_JSON_RETRY_DELAY_FACTOR = 0.4\n+\n # Google Drive API imports\n from google.oauth2.credentials import Credentials\n from google_auth_oauthlib.flow import InstalledAppFlow\n@@ -467,7 +469,7 @@\n             status = getattr(e, \"status\", None)\n             is_conflict = status == 409 or \"does not match\" in str(e).lower()\n             if is_conflict and attempt < max_retries:\n-                delay = 0.4 * attempt\n+                delay = VIDEOS_JSON_RETRY_DELAY_FACTOR * attempt\n                 logger.warning(\n                     f\"Conflict while upserting videos.json (attempt {attempt}/{max_retries}). \"\n                     f\"Retrying in {delay:.1f}s...\"\n"}]},{"message":"The magic number 2.0 for delay calculation is unexplained. Consider defining this as a named constant (e.g., `ENTRY_VERIFICATION_DELAY_FACTOR = 2.0`) to clarify the retry backoff strategy.","fixFiles":[{"filePath":"gh.py","diff":"diff --git a/gh.py b/gh.py\n--- a/gh.py\n+++ b/gh.py\n@@ -8,6 +8,8 @@\n from typing import Tuple, Optional, Callable\n from env_config import get_env\n \n+ENTRY_VERIFICATION_DELAY_FACTOR = 2.0\n+\n # Google Drive API imports\n from google.oauth2.credentials import Credentials\n from google_auth_oauthlib.flow import InstalledAppFlow\n@@ -690,7 +692,...