Add graceful shutdown, logging, streaming with Range Requests, and state.json cache

- Graceful shutdown: SIGTERM/SIGINT handlers stop continuous scraper,
  disconnect Telegram client, drain running jobs with timeout
- Logging: replace print() with logging module throughout web server,
  basicConfig in main.py with structured log format
- Streaming: chunked file reads (64KB) instead of loading entire files
  into memory; full HTTP Range Request support (206 Partial Content)
  for video seeking in the browser
- Cache: TTL-based in-memory cache (1s) for state.json reads in
  StateStore, thread-safe with existing RLock
- Bugfix: send_error_json now converts HTTPStatus to int for JSON

main.py

@@ -1,7 +1,16 @@
+import logging
+import sys
+
 from webui_server import run_server
 
 
 def main():
+    logging.basicConfig(
+        level=logging.INFO,
+        format="%(asctime)s [%(levelname)s] %(name)s: %(message)s",
+        datefmt="%Y-%m-%d %H:%M:%S",
+        stream=sys.stdout,
+    )
     run_server()