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.

backend/internal/engine/engine.go

@@ -17,7 +17,7 @@ import (
 	"errors"
 	"fmt"
 
-	"scrabble-solver/rules"
+	"gitea.iliadenisov.ru/developer/scrabble-solver/rules"
 )
 
 // Variant identifies a Scrabble variant the backend offers. Each maps to a