feat(auth-jwt): initial implementation — JWT lifecycle and AuthMiddleware (v1.0.0)

Introduces code.nochebuena.dev/einherjar/auth-jwt — JWT authentication middleware
and token lifecycle management for the Einherjar framework. Absorbs httpauth-jwt
from micro-lib with three changes: logger parameter on AuthMiddleware, httputil.Error
for consistent 401 responses, and added ECDSA support.

Signers and Verifiers (one file per implementation — CT-6 compliant):
- Verifier interface — Verify(tokenString string) (*jwt.Token, error)
- Signer interface — extends Verifier; adds Sign(claims jwt.Claims) (string, error)
- signer_hmac.go — NewHMACSigner(secret) → HS256; jwt.WithJSONNumber() on Verify
- signer_rsa.go — NewRSASigner(key) + NewRSASignerFromPEM(pem) → RS256
- verifier_rsa.go — NewRSAPublicKeyVerifier + NewRSAPublicKeyVerifierFromPEM → RS256 verify-only
- signer_ec.go — NewECSigner(key) + NewECSignerFromPEM(pem) → ES256/384/512; algorithm
  auto-detected from key curve (P-256→ES256, P-384→ES384, P-521→ES512)
- verifier_ec.go — NewECPublicKeyVerifier + NewECPublicKeyVerifierFromPEM → EC verify-only

Token lifecycle:
- TokenConfig struct — AccessTTL, RefreshTTL, Issuer
- TokenPair struct — AccessToken, RefreshToken, ExpiresIn
- IssueTokenPair — access + refresh pair; customClaims merged at top level; refresh
  carries only sub/iss/iat/exp/jti/fam; jwt.WithJSONNumber() preserves int64 bitmasks
- Blacklist interface — IsRevoked + Revoke; satisfied by cache-valkey via duck typing
- ErrTokenRevoked — errors.New sentinel; errors.Is pattern for replay-attack detection
- RefreshTokenPair — verifies token, checks blacklist, revokes old JTI, issues new pair

HTTP middleware:
- AuthMiddleware(logger, verifier, publicPaths) — verifies Bearer tokens; calls
  authmw.SetTokenData on success; 401 routed through httputil.Error (Warn level);
  publicPaths use path.Match wildcards; accepts Verifier (not Signer) to enforce
  narrowest-interface principle for verify-only services

Compliance test (package authjwt_test) enforces CT-6 (≤1 exported TypeSpec/file),
compile-time interface satisfaction, and behavioural coverage: HMAC/RSA/EC sign+verify,
algorithm mismatch rejection, IssueTokenPair claims/jti/fam, RefreshTokenPair success/
revoked/blacklist-error/custom-claims, MaxInt64 json.Number precision, AuthMiddleware
valid/invalid/expired/missing/public-path/wildcard/JSON-body/RSA-verifier/SetTokenData.

Depends on auth v1.0.0, contracts v1.0.0, core v1.0.0, web v1.0.0, jwt/v5 v5.2.1.

- identifiable.go: package-level Module variable (observability.Identifiable) for version
  identification — auth-jwt is a function library; not registered with the launcher

signer_rsa.go

@@ -0,0 +1,57 @@
+package authjwt
+
+import (
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+var _ Signer = (*rsaSigner)(nil)
+
+type rsaSigner struct {
+	private *rsa.PrivateKey
+	public  *rsa.PublicKey
+}
+
+// NewRSASigner returns a Signer backed by RSA-SHA256 (RS256).
+// The public key is derived from the private key — no separate argument needed.
+func NewRSASigner(privateKey *rsa.PrivateKey) Signer {
+	return &rsaSigner{private: privateKey, public: &privateKey.PublicKey}
+}
+
+// NewRSASignerFromPEM parses a PKCS#8 or PKCS#1 PEM-encoded RSA private key.
+func NewRSASignerFromPEM(pemKey []byte) (Signer, error) {
+	block, _ := pem.Decode(pemKey)
+	if block == nil {
+		return nil, fmt.Errorf("no PEM block found")
+	}
+	key, err := x509.ParsePKCS8PrivateKey(block.Bytes)
+	if err != nil {
+		rsaKey, err2 := x509.ParsePKCS1PrivateKey(block.Bytes)
+		if err2 != nil {
+			return nil, fmt.Errorf("parse RSA private key: %w", err)
+		}
+		return NewRSASigner(rsaKey), nil
+	}
+	rsaKey, ok := key.(*rsa.PrivateKey)
+	if !ok {
+		return nil, fmt.Errorf("PEM key is not an RSA private key")
+	}
+	return NewRSASigner(rsaKey), nil
+}
+
+func (s *rsaSigner) Sign(claims jwt.Claims) (string, error) {
+	return jwt.NewWithClaims(jwt.SigningMethodRS256, claims).SignedString(s.private)
+}
+
+func (s *rsaSigner) Verify(tokenString string) (*jwt.Token, error) {
+	return jwt.Parse(tokenString, func(t *jwt.Token) (any, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodRSA); !ok {
+			return nil, fmt.Errorf("unexpected signing method %q", t.Header["alg"])
+		}
+		return s.public, nil
+	}, jwt.WithJSONNumber())
+}