fix: unify NIP-OA relay membership enforcement across all ingress paths

[tlongwell-block]

Summary

When SPROUT_REQUIRE_RELAY_MEMBERSHIP=true, agents whose owner is a relay member should be allowed to connect via NIP-OA attestation. Previously this fallback only worked for media and git transport endpoints — WebSocket NIP-42 auth, the HTTP bridge, and the audio WebSocket handler all lacked it.

This patch removes the duplicate enforce_ws_relay_membership() function and routes all ingress paths through the single shared enforce_relay_membership() helper that already supports NIP-OA owner-delegation.

What Changed

Ingress Path	Before	After
WebSocket NIP-42 (handlers/auth.rs)	Separate function, no NIP-OA	Shared helper with auth tag from signed AUTH event
HTTP bridge /events, /query, /count (bridge.rs)	Passed None for auth tag	Reads x-auth-tag header
Audio WebSocket (audio/handler.rs)	Passed None for auth tag	Extracts auth tag from AUTH event
MCP client (relay_client.rs)	Had auth_tag but never sent it in NIP-42	Includes tag in all AUTH events (connect + reconnect + mid-session)
MCP media upload (upload.rs)	No x-auth-tag header	Sends header on PUT requests

Security Properties

• Fail-closed: DB errors, missing tags, invalid signatures all deny access
• WS path: auth tag is integrity-protected by the AUTH event's Schnorr signature — tampering fails NIP-42 verification before we ever inspect it
• HTTP path: verify_auth_tag binds the attestation to the NIP-98-authenticated agent pubkey — cannot replay another agent's tag
• Spec conformance: events with >1 auth tag are treated as having no valid tag (per NIP-OA spec)
• No new attack surface: the only new accept path is "valid NIP-OA Schnorr sig + owner is relay member"

Unified Flow

All Ingress → transport auth → extract auth tag → enforce_relay_membership()
                                                         │
                                          ┌──────────────┼──────────────┐
                                          ▼              ▼              ▼
                                    !require_membership  direct member  NIP-OA valid
                                       → ALLOW           → ALLOW       + owner is member
                                                                       → ALLOW
                                          else → DENY (fail-closed)

Testing

• cargo build --workspace ✅
• cargo clippy --workspace --all-targets -- -D warnings ✅
• cargo fmt --all -- --check ✅
• cargo test --workspace ✅ (all 1,251 tests pass)

Stats

6 files changed, +116/−68 (net code reduction from deleting the duplicate function)