feat(cache): add channel info cache with TTL and throttling

Introduce `cached_get_chat` in `tg_cache` to store channel metadata on disk with a configurable TTL (default 12 hours). Use the `tg_rpc` throttling context for RPC calls. Update `rss_generator` to use the cached version and document related environment variables in `dockercompose.yml`.

rss_generator.py

@@ -344,7 +344,8 @@ async def generate_channel_rss(channel: str | int,
         channel_info_start_time = time.time()
         try:
             channel  = post_parser.channel_name_prepare(channel)
-            channel_info = await post_parser.client.get_chat(channel)
+            from tg_cache import cached_get_chat
+            channel_info = await cached_get_chat(post_parser.client, channel)
             channel_title = channel_info.title or f"Telegram: {channel}"
             channel_username = channel_info.username or (str(channel_info.id) if channel_info.id and str(channel_info.id).startswith('-100') else None)
             if not channel_username:
@@ -556,7 +557,8 @@ async def generate_channel_html(channel: str | int,
         try:
             channel = post_parser.channel_name_prepare(channel)
             logger.debug(f"Prepared channel identifier for HTML: {channel} (type: {type(channel)})") # Log prepared channel
-            channel_info = await post_parser.client.get_chat(channel)
+            from tg_cache import cached_get_chat
+            channel_info = await cached_get_chat(post_parser.client, channel)
             channel_username = channel_info.username or (str(channel_info.id) if channel_info.id and str(channel_info.id).startswith('-100') else None)
             if not channel_username:
                 logger.warning(f"Could not get username for channel {channel} in HTML generation, returning error feed structure (as string). NOTE: This should ideally return HTML error page.")