fix(langfuse): generate a 64-character hex encryption key

[andrasbacsai]

Summary

• Langfuse needs ENCRYPTION_KEY to be 64 hex characters (256 bits). The template set it from SERVICE_PASSWORD_64_LANGFUSE, which produces a 64-character password with non-hex characters. Langfuse rejected that value, so adding an LLM connection (for example, an OpenAI API key) failed with an invalid key length error.
• ENCRYPTION_KEY now comes from SERVICE_HEX_64_ENCRYPTIONKEY, which generates a valid 64-character hex string. This matches the manual workaround from the issue (openssl rand -hex 32).
• Rebuilt the base64-encoded Langfuse compose in service-templates.json and service-templates-latest.json.

fixes #8797

Notes

• The change applies to new Langfuse deployments.
• Existing deployments that already hit this error can replace ENCRYPTION_KEY with the output of openssl rand -hex 32. Any secrets Langfuse encrypted with the old key (such as saved LLM connection keys) may need to be entered again.

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Fixes #8797