Feature: Complete JWE support (RSA-OAEP, ECDH-ES, ES512)

[hahwul]

Summary

Current JWE support is demo-level (encode_jwe_demo). Need full JWE suite:

• Key management algorithms: RSA-OAEP, RSA-OAEP-256, ECDH-ES, ECDH-ES+A128KW, A128KW, A256KW
• ES512 (P-521) algorithm support for JWT signing
• Proper JWE round-trip (encode → decode → verify)

Related

• PR Add JWE attack capabilities: decryption, key cracking, and vulnerability detection #175 laid the groundwork with JWE attack features
• josekit dependency already supports these algorithms