SEP-1841: Correct sidecar README guidance on when to set SEP_INTERNAL_TOKEN - #1372

The SEP_INTERNAL_TOKEN row in sidecar/README.md's deployment-inputs table referenced PMM's nginx overlay as the trigger for setting the variable explicitly. That overlay was retired in SEP-1795/SEP-1810 and no longer exists.

• Replaced the stale "when PMM's nginx overlay pins a specific value" sentence with the condition grounded in shipped code: rotating the token independently of SECRET_KEY (per derive_internal_token in app/core/config.py)
• Added context on what the token authenticates (service-to-service calls, e.g. scheduled inventory sync)
• Preserved the HMAC-derivation sentence

Before:

| SEP_INTERNAL_TOKEN | no | Derived from SECRET_KEY by HMAC when unset. Set it explicitly when PMM's nginx overlay pins a specific value. |

After:

| SEP_INTERNAL_TOKEN | no | Authenticates SEP-internal service-to-service calls, such as the scheduled inventory sync. Derived from SECRET_KEY by HMAC when unset, so every process sharing the key resolves the same token. Set it explicitly only to rotate it independently of SECRET_KEY. |