docs: rename GOOGLE_OAUTH.md → AUTHENTICATION.md, update references

CLAUDE.md

@@ -89,7 +89,7 @@ python3 -c "import secrets; print(secrets.token_hex(32))"
 
 **Omit all auth vars to run without auth** (default — open server).
 
-**Full guide:** [`docs/GOOGLE_OAUTH.md`](docs/GOOGLE_OAUTH.md)
+**Full guide:** [`docs/AUTHENTICATION.md`](docs/AUTHENTICATION.md)
 
 ## Architecture
 

README.md

@@ -282,7 +282,7 @@ Clients present it as `Authorization: Bearer <UNRAID_MCP_API_KEY>`. Set both `GO
 
 Omit both to run without authentication (default — open server).
 
-**Full guide:** [`docs/GOOGLE_OAUTH.md`](docs/GOOGLE_OAUTH.md)
+**Full guide:** [`docs/AUTHENTICATION.md`](docs/AUTHENTICATION.md)
 
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