feat(api): detect UCAN session on nostr-login connect page

[sergey3bv]

Summary

• GET /connect/*nostrconnect now validates Keycast's session, checks the user exists for the tenant;
• Integration tests cover.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• Solving TODOs left in the code.

Related Issue

• Closes feat: Detect Keycast session on nostr-login connect GET page #226

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved