Mint and verify short-lived RS256 user tokens.

ExApps sign after AppAPI auth; Microservices check a static public key or an OIDC issuer and forward the same bearer.

Co-authored-by: Cursor <cursoragent@cursor.com>

CONTEXT.md

@@ -0,0 +1,25 @@
+# go-usertoken
+
+User tokens an ExApp mints for Microservices. Import `gitea.neitzel.de/konrad/go-usertoken`.
+
+## Language
+
+**User token**:
+A short-lived RS256 JWT whose `sub` is the Nextcloud user id. Optional `groups` is a snapshot taken when the ExApp minted it. Microservices forward the same token to each other.
+_Avoid_: AppAPI secret, access token (too broad), session
+
+**Caller**:
+The user id, username, and optional groups read from a verified user token.
+_Avoid_: Requesting user (that is the AppAPI name, before a token exists), principal
+
+**Signer**:
+The ExApp-side minter. It holds the only private key.
+_Avoid_: issuer (the `iss` string), identity server
+
+**Static key**:
+The verify mode that checks a user token with a configured RSA public key and a fixed `iss` string. No discovery URL.
+_Avoid_: JWKS, OIDC
+
+**OIDC issuer**:
+The verify mode that discovers keys at an identity server (for example Keycloak). One Microservice process uses this mode or static key, not both.
+_Avoid_: running discovery on the ExApp