Add Ed25519 challenge-response authentication

[oetiker]

Summary

Adds Ed25519 challenge-response authentication as an alternative to API key auth, giving each device a unique cryptographic identity.

Why this matters

API keys are shared secrets — if one is intercepted, an attacker can impersonate the device indefinitely. Ed25519 authentication eliminates this class of attack:

• No shared secrets on the wire. The private key never leaves the device. Only the public key and a timestamped signature are transmitted.
• Replay-resistant. Each request is signed with a server-provided timestamp, so captured requests cannot be reused outside the validity window.
• Tamper-evident identity. The keypair is generated on-device and stored in NVS. A device's public key becomes its unforgeable identity — the server can detect if a device has been cloned or factory-reset.
• One additional round-trip. The device fetches the server timestamp via /api/time before signing. This ensures clock-independent replay protection.
• Server-controlled rollout. The server sets auth_mode per device via /api/setup, so existing devices continue working unchanged.

What changed

• Ed25519 keypair generation and NVS persistence (device_identity.cpp)
• Challenge-response signing: sign(timestamp_ms || public_key) (auth_signature.cpp)
• TweetNaCl Ed25519 implementation with hardening: signature malleability rejection (S < L check), VLA stack guard
• Key generation moved after WiFi connect for RF-based esp_random() entropy
• API clients send X-Public-Key, X-Signature, X-Timestamp headers when in ed25519 mode
• Shared bytesToHex extracted to hex_utils.h

Test plan

• pio run builds cleanly
• Device generates and persists keypair on first boot
• X-Public-Key, X-Signature, X-Timestamp headers sent when auth_mode: "ed25519"
• Factory reset regenerates keypair
• Fallback to API key auth when server returns auth_mode: "api_key"

🤖 Generated with Claude Code

[Fr3dr1ckson]

Hey, thanks for helping improve the firmware! This feature adds quite a bit to review, so it’ll take me some time to dig into it. I’ll post an update once I start working on it.