docs: normalize remaining streaming plan-name references

Replace "Reservoir Mastery" in the README streaming section and the
reject_subscription ConnectionError message with current billing
language: "Professional plan ($99/mo) or higher" (streaming gates at
Professional+). The error string keeps the "Subscription rejected"
prefix so the existing streaming unit test still matches.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: karlwaldman

README.md

@@ -535,7 +535,7 @@ prices = asyncio.run(get_prices())
 ## 📡 Real-Time WebSocket Streaming (New in v1.8.0)
 
 Stream live oil and energy prices over WebSocket instead of polling. Streaming
-is a **Professional / Reservoir Mastery** feature and is exposed through the
+is a **Professional plan ($99/mo) or higher** feature and is exposed through the
 async client via `client.stream`.
 
 Install the optional `stream` extra:

oilpriceapi/streaming/client.py

@@ -155,7 +155,7 @@ async def _subscribe(self) -> None:
             if msg_type == "reject_subscription":
                 raise ConnectionError(
                     "Subscription rejected — check your plan tier and API key "
-                    "(WebSocket streaming requires Reservoir Mastery)."
+                    "(WebSocket streaming requires the Professional plan ($99/mo) or higher)."
                 )
             # Ignore pings / pre-confirmation noise.