LINUS ROUND 4: RS256 auth + ouroboros-identity integration

- auth/rs256.go: RS256 JWT validation with AAMOS public key
- auth/rs256_test.go: 4 RS256 tests (success, invalid sig, expired, HS256 reject)
- auth/integration_test.go: Real AAMOS identity service integration test
- Copied jwt-public.pem from /opt/amos/data/keys/
- ouroboros-identity running on port 3208

backend/auth/jwt-public.pem

@@ -0,0 +1,9 @@
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAnwnzpuZ5W64u5ScP/g9d
+SCDVnMp/oYLlX0iYIKR9OBW3KjmYASdwumtmGT4ssz2yENAHjBR24hr9EP5oCbpv
+kvU/e9mM+Rx2POky9Ld/RyhDZRMpLlz/hNHDbt1x/4jPQNy7cb+0o4fjqPB/UXFd
+8YEJKnTD+BLsmUpIf7P0pFASLXi8BCWI1w26BwD0iAjcIQbllzdfsLfO4lFn9z0c
+RprzZGLlOpwCslfvNFrz6vB9HnUxYHIPexB54YwTtUZjpoz+Um/A5y6nAn94P/E5
+RqTqVp80vHPpTXL/KSOwU6E8NQYHWPhp1eziiq0hfTOZeDzZIeDKn+tHNwBiU71q
+KQIDAQAB
+-----END PUBLIC KEY-----