feat(auth): surface debuggable reasons for rejected issuer/session tokens

[jeroenrinzema]

Problem

When authenticating with a trusted issuer (or a Lunogram session), any token failure collapsed into a single bare unauthorized. If a token was missing data such as the exp claim, the caller got the same 401 as a wrong signing key — with no way to tell which, making integrations painful to debug.

Change

Surface a precise, fixable reason in the 401 — but only once the token has proven to be authentically ours, so we never leak whether an issuer or project exists.

Trusted issuer (trusted_issuer.go)

• A descriptive reason is returned only after the token's iss resolves to a configured trusted issuer. Failures before that (empty/unparseable token, unknown issuer) stay a generic ErrUnauthorized — they'd otherwise leak issuer/project existence.
• verifyTrustedIssuerToken now preserves the underlying golang-jwt cause instead of flattening it, and describeTokenError maps it to a safe message (missing claim, expired, not-yet-valid, issuer/audience mismatch). Signature / malformed / key-resolution failures collapse to a generic reason so verification internals never leak.

Session (session.go)

• A descriptive reason is returned only after the ES256 signature verifies. golang-jwt validates claims strictly after the signature, so a claim-level error (ErrTokenInvalidClaims) proves the token is one of ours.
• Signature / signing-method / format failures stay ErrUnauthorized so the auth chain still falls through to the trusted-issuer handler (a real trusted-issuer JWT is RS256 and must not be mis-described as a session token).

Safety / compatibility

• HTTP status is unchanged — still 401 via SecurityRequirementsError. Only the problem detail gains a specific message, e.g. token is missing the required "exp" claim or token has expired.
• The descriptive errors deliberately do not satisfy errors.Is(err, ErrUnauthorized), so the middleware returns them instead of collapsing to a bare unauthorized. This only short-circuits the chain for tokens already proven to belong to that scheme, so no request that currently succeeds can regress.

Tests

Updated and extended trusted_issuer_middleware_test.go and session_middleware_test.go to cover the new reasons (missing exp, expired, wrong key, wrong/missing issuer, missing/invalid amid, missing sub, unknown session method) and to assert that signature/format failures still stay generic and fall through.