feat: add support for PROXY protocol v2

[molivier]

Add full support for PROXY protocol version 2 to enable proper client
IP forwarding from load balancers and reverse proxies.

Changes:

• Implement PROXY protocol v2 parser and handler (src/proxy.rs)
• Make ppv2 mode required when specified via configuration
• Refactor proof-of-work validation for better maintainability
• Update handler to check for local addresses with ppv2
• Apply Clippy recommendations and code cleanup

The PROXY protocol support allows sossette to correctly identify
client IP addresses when deployed behind TCP load balancers.

Security Note: When enabling PROXY protocol, ensure it's only accepted
from trusted sources (load balancers/proxies). Misconfiguration may
expose the application to IP spoofing. Deployers are responsible for
proper network segmentation and access control to prevent leakage of
client information or unauthorized protocol injection.

[erdnaxe]

Manually rebased and merged in 422d938