Phase 4: delete the gRPC server, gateway, proto/buf toolchain, second port, and CI codegen

[SyniRon]

Parent

Part of #112 (PRD: Goodbye gRPC) — Phase 4 (Delete).

What to build

The sweep, after cutover has soaked: delete the proto files, buf configuration, all codegen artifacts (generated Go, gateway code, the generated Swagger 2.0 assets), the gRPC server and gateway packages, the second-port deployment/reverse-proxy config, the CI codegen steps (buf install, generate, the release-time proto version sed), and the build tooling that exists only for them. Retire the proto-coupled tests that guard deleted machinery (the deprecation contract, the generated-spec assertions). Retire the throwaway request-log analytics — Prometheus owns those questions now, and the temporary duration= obligation ends with the old stack (the new stack's request logging is unconstrained by the old format).

go mod tidy — the PRD sizes the dependency shrink at roughly a hundred modules.

Verification mirrors the PRD: build green with no buf/protoc anywhere, golden suite green (the proof the public contract survived the deletion), dependency tree visibly smaller.

Workflow: the suites are the net — nothing green may go red by deletion.

Acceptance criteria

• No proto/buf/codegen in repo, CI, or build tooling; build + test green without downloading any of it
• gRPC server + gateway packages gone; one listener in code and in config
• Golden suite green post-deletion
• Log-scraping analytics retired
• Dependency delta reported in the PR description

Blocked by

• Phase 3 cutover: single listener, datastore type-swap, Keycloak removal, deliberate breaks live #134 — cutover, plus its soak