refactor(yaad): split rest.go into handlers and extra files

Mechanically split internal/server/rest.go (1370 LOC) into three
same-package files, moving code verbatim with no behavior or API changes:

- rest.go: server lifecycle, middleware, route registration, HTTP helpers
- rest_handlers.go: core memory/graph/node/session/maintenance handlers
- rest_extra.go: export/import, skill, version-history, confidence,
  compression, bridge, and advanced-feature handlers

Verified with go build, go vet, go test, and golangci-lint (0 issues).

Author: Patel230