feat(api): implement API key management and authentication enhancements

- Added endpoints for managing API keys, including creation, retrieval, updating, and revocation.
- Introduced a new Auth session endpoint to retrieve current tenant and role information.
- Updated the authentication middleware to support API key-based authentication and track last used timestamps.
- Enhanced documentation to reflect new API key functionalities and usage guidelines.
- Improved logging for demo authentication scenarios.

internal/authkey/token.go

@@ -0,0 +1,34 @@
+// Package authkey generates API tokens and derives lookup hashes (no persistence).
+package authkey
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+)
+
+const tokenPrefix = "evobgp_"
+
+// GenerateToken returns a new bearer token (evobgp_ + 32 random bytes, base64url).
+func GenerateToken() (string, error) {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		return "", fmt.Errorf("authkey: generate token: %w", err)
+	}
+	return tokenPrefix + base64.RawURLEncoding.EncodeToString(b), nil
+}
+
+// HashToken returns SHA-256 of the full token (32 bytes).
+func HashToken(token string) []byte {
+	sum := sha256.Sum256([]byte(token))
+	return sum[:]
+}
+
+// Prefix returns the first 8 characters of the token for display.
+func Prefix(token string) string {
+	if len(token) <= 8 {
+		return token
+	}
+	return token[:8]
+}