Add comprehensive config validation and race testing target

Split config validation into validateFields() (pure logic) and
validateFiles() (filesystem checks) for testability. New validations:
TLS file existence, token TTL parseability/positivity, Argon2 params > 0,
valid log level, non-empty listen addresses. Added 18 tests covering all
validation paths. Added `make test-race` target. Resolves A-015 and A-017.

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

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build test vet lint proto proto-lint clean all
+.PHONY: build test test-race vet lint proto proto-lint clean all
 
 LDFLAGS := -trimpath -ldflags="-s -w -X main.version=$(shell git describe --tags --always --dirty 2>/dev/null || echo dev)"
 
@@ -11,6 +11,9 @@ build:
 test:
 	go test ./...
 
+test-race:
+	go test -race ./...
+
 vet:
 	go vet ./...