feat(rust): replace python hardware identity with secure ed25519-dalek implementation

[Tenerife-Q]

Motivation

The official GSoC roadmap explicitly targets migrating the camera daemon to Rust and establishing "secure device cryptographic key generation using hardware entropy". Currently, hardware_identity.py handles this in Python, which is not memory-safe and less suitable for strict cryptographic device sealing.

Implementation

This PR sets up the foundational lensmint-identity Rust crate:

• Drops the Python generation in favor of ed25519-dalek (v2).
• Uses rand_core::OsRng to source true hardware-level entropy directly from the OS (/dev/urandom on Raspberry Pi) to guarantee uncloneable device keys.
• Provides a clean, safe abstraction (DeviceIdentity) to extract the camera ID (hex public key) and sign metadata payloads.

Testing

Added unit tests to verify:

• Hex encoding formatting for the 32-byte public key (Camera ID).
• Signature generation and verification over a mock perceptual hash payload.

This PoC lays the groundwork for the broader Rust daemon rewrite planned for the summer.