Stage 14: solver & dictionary split — consume published module + DAWG artifact (TODO-1/TODO-2)

- backend/go.mod pins gitea.iliadenisov.ru/developer/scrabble-solver v1.0.0; the engine's
  imports use the published module path; go.work drops the solver replace (GOPRIVATE fetches
  it directly from Gitea). The solver's wordlist/dictdawg are now public packages.
- CI (go-unit, integration): drop the solver sibling-clone, set GOPRIVATE, and download the
  dictionary DAWG release artifact (scrabble-dawg-<DICT_VERSION>.tar.gz from the new
  scrabble-dictionary repo) for BACKEND_DICT_DIR.
- Docs: ARCHITECTURE §5/§11/§13/§14 + backend/README updated to the published-module +
  release-artifact model. PLAN.md re-scoped Stage 14 to the split and added Stages 15 (deploy
  infra & test contour), 16 (prod contour), 17 (dual Telegram bots); TODO-1/TODO-2 marked done.

go.work.sum

@@ -1,6 +1,8 @@
 cel.dev/expr v0.25.1/go.mod h1:hrXvqGP6G6gyx8UAHSHJ5RGk//1Oj5nXQ2NI02Nrsg4=
 cloud.google.com/go/compute/metadata v0.9.0/go.mod h1:E0bWwX5wTnLPedCKqk3pJmVgCBSM6qQI1yTBdEb3C10=
 filippo.io/edwards25519 v1.2.0/go.mod h1:xzAOLCNug/yB62zG1bQ8uziwrIqIuxhctzJT18Q77mc=
+gitea.iliadenisov.ru/developer/scrabble-solver v1.0.0 h1:ntN6m4cOB+4FelleO2nkAIZp8WSc+v25neetzfdUuuw=
+gitea.iliadenisov.ru/developer/scrabble-solver v1.0.0/go.mod h1:G60OiGZtkrRyYX8P3SSsjVpU707fufmZkvCkNFPFWrY=
 github.com/ClickHouse/ch-go v0.71.0/go.mod h1:NwbNc+7jaqfY58dmdDUbG4Jl22vThgx1cYjBw0vtgXw=
 github.com/ClickHouse/clickhouse-go/v2 v2.45.0/go.mod h1:giJfUVlMkcfUEPVfRpt51zZaGEx9i17gCos8gBl392c=
 github.com/GoogleCloudPlatform/opentelemetry-operations-go/detectors/gcp v1.31.0/go.mod h1:P4WPRUkOhJC13W//jWpyfJNDAIpvRbAUIYLX/4jtlE0=