feat(MM-97): implement GET /xrpc/com.atproto.identity.resolveHandle #28

Summary#

Implements GET /xrpc/com.atproto.identity.resolveHandle?handle=<handle> (no auth required)
Resolution order: local handles table → DNS TXT record at _atproto.<handle> → 404 HandleNotFound
Adds TxtResolver trait to dns.rs (injectable, follows DnsProvider pattern) with HickoryTxtResolver production impl (hickory-resolver 0.25)
Adds HandleNotFound (404) to ErrorCode
Adds txt_resolver: Option<Arc<dyn TxtResolver>> to AppState; wired via HickoryTxtResolver::from_system_conf() in main.rs

cargo test — 215 tests pass (6 new: local lookup, DNS fallback, DNS no-match, no resolver, empty DNS response, content-type)
cargo clippy --workspace -- -D warnings — clean
cargo fmt --all --check — clean
Bruno: GET /xrpc/com.atproto.identity.resolveHandle?handle=<local-handle> returns { "did": "..." }
Bruno: unknown handle returns { "error": { "code": "HANDLE_NOT_FOUND" } } with 404