fix(server): improve error message for encrypted TLS private keys (#4742)

Added TLS certificate validation that detects encrypted (password-protected)
private keys and provides a clear error message with instructions on how to
decrypt them using openssl. This addresses user confusion when Go's standard
library fails with the cryptic 'tls: failed to parse private key' error.

Changes:
- Added validateTLSCertificates function to validate certs before server start
- Added isEncryptedPEM helper to detect both PKCS#8 and legacy encrypted keys
- Added comprehensive tests for TLS validation including encrypted key detection
- Added integration test that starts server with TLS and verifies HTTPS works
- Added test certificates (valid for 100 years) with SAN for localhost

Signed-off-by: Deluan <deluan@navidrome.org>

server/testdata/test_cert_encrypted.pem

@@ -0,0 +1,22 @@
+-----BEGIN CERTIFICATE-----
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+BQAwYjELMAkGA1UEBhMCVVMxDTALBgNVBAgMBFRlc3QxDTALBgNVBAcMBFRlc3Qx
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+Hy1L5a2vUDbsdONdvRkYZRHqMpG4bdD8D3j2
+-----END CERTIFICATE-----