Phase 5: hardening — gRPC, observability, auth, config

- proto: lakehouse.proto with CatalogService, QueryService, StorageService, AiService
- proto crate: tonic-build codegen from proto definitions
- catalogd: gRPC CatalogService implementation
- gateway: dual HTTP (:3100) + gRPC (:3101) servers
- gateway: OpenTelemetry tracing with stdout exporter
- gateway: API key auth middleware (toggleable)
- shared: TOML config system with typed structs and defaults
- lakehouse.toml config file
- ADR-006 and ADR-007 documented

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/catalogd/Cargo.toml

@@ -15,3 +15,5 @@ bytes = { workspace = true }
 chrono = { workspace = true }
 uuid = { workspace = true }
 object_store = { workspace = true }
+proto = { path = "../proto" }
+tonic = { workspace = true }