Add random 4-digit TV token per child for obfuscated TV URLs

Each child is assigned a unique permanent tv_token on creation. The TV
dashboard URL (/tv/:tvToken) and WebSocket (/ws/:tvToken) now use this
token instead of the internal DB ID. Existing children are backfilled
on startup. README updated to reflect the change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/child.py

@@ -23,5 +23,6 @@ class ChildOut(BaseModel):
     is_active: bool
     color: str
     strikes: int = 0
+    tv_token: int | None = None
 
     model_config = {"from_attributes": True}