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_ec.go

@@ -0,0 +1,66 @@
+package authjwt
+
+import (
+	"crypto/ecdsa"
+	"crypto/x509"
+	"encoding/pem"
+	"fmt"
+
+	"github.com/golang-jwt/jwt/v5"
+)
+
+var _ Signer = (*ecSigner)(nil)
+
+type ecSigner struct {
+	private *ecdsa.PrivateKey
+	public  *ecdsa.PublicKey
+}
+
+// NewECSigner returns a Signer backed by ECDSA.
+// The signing algorithm is auto-detected from the key's curve:
+// P-256→ES256, P-384→ES384, P-521→ES512.
+func NewECSigner(privateKey *ecdsa.PrivateKey) Signer {
+	return &ecSigner{private: privateKey, public: &privateKey.PublicKey}
+}
+
+// NewECSignerFromPEM parses a PKCS#8 PEM-encoded ECDSA private key.
+func NewECSignerFromPEM(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 {
+		return nil, fmt.Errorf("parse EC private key: %w", err)
+	}
+	ecKey, ok := key.(*ecdsa.PrivateKey)
+	if !ok {
+		return nil, fmt.Errorf("PEM key is not an EC private key")
+	}
+	return NewECSigner(ecKey), nil
+}
+
+func (s *ecSigner) Sign(claims jwt.Claims) (string, error) {
+	return jwt.NewWithClaims(ecAlg(s.private), claims).SignedString(s.private)
+}
+
+func (s *ecSigner) Verify(tokenString string) (*jwt.Token, error) {
+	alg := ecAlg(s.private)
+	return jwt.Parse(tokenString, func(t *jwt.Token) (any, error) {
+		if t.Method.Alg() != alg.Alg() {
+			return nil, fmt.Errorf("unexpected signing method %q", t.Header["alg"])
+		}
+		return s.public, nil
+	}, jwt.WithJSONNumber())
+}
+
+func ecAlg(key *ecdsa.PrivateKey) *jwt.SigningMethodECDSA {
+	switch key.Curve.Params().Name {
+	case "P-384":
+		return jwt.SigningMethodES384
+	case "P-521":
+		return jwt.SigningMethodES512
+	default:
+		return jwt.SigningMethodES256
+	}
+}