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

app_state.py

@@ -1,7 +1,8 @@
 import json
 import threading
+import time
 from pathlib import Path
-from typing import Any, Callable, Dict
+from typing import Any, Callable, Dict, Optional
 
 
 DEFAULT_STATE: Dict[str, Any] = {
@@ -24,17 +25,32 @@ class StateStore:
     def __init__(self, path: Path):
         self.path = path
         self.lock = threading.RLock()
+        self._cache: Optional[Dict[str, Any]] = None
+        self._cache_time: float = 0
+        self._cache_ttl: float = 1.0
 
     def load(self) -> Dict[str, Any]:
         with self.lock:
+            now = time.time()
+            if self._cache is not None and (now - self._cache_time) < self._cache_ttl:
+                return dict(self._cache)
             if not self.path.exists():
-                return self._default_state()
+                result = self._default_state()
+                self._cache = result
+                self._cache_time = now
+                return result
             try:
                 with self.path.open("r", encoding="utf-8") as handle:
                     state = json.load(handle)
             except (json.JSONDecodeError, OSError):
-                return self._default_state()
-            return self._merge_defaults(state)
+                result = self._default_state()
+                self._cache = result
+                self._cache_time = now
+                return result
+            result = self._merge_defaults(state)
+            self._cache = result
+            self._cache_time = now
+            return result
 
     def save(self, state: Dict[str, Any]) -> None:
         with self.lock:
@@ -44,6 +60,7 @@ class StateStore:
                 json.dump(self._merge_defaults(state), handle, ensure_ascii=False, indent=2)
                 handle.write("\n")
             tmp_path.replace(self.path)
+            self._cache = None
 
     def update(self, mutator: Callable[[Dict[str, Any]], None]) -> Dict[str, Any]:
         with self.lock: