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

@@ -8,11 +8,11 @@ use (
 	./platform/telegram
 )
 
-// The scrabble-solver engine is consumed in-process as a library. Its module
-// path is the bare "scrabble-solver" (not a URL), so it cannot be fetched as a
-// versioned dependency via VCS; the workspace points it at the sibling checkout.
-// CI clones that sibling next to this repository before building.
-replace scrabble-solver => ../scrabble-solver
+// The scrabble-solver engine is consumed in-process as a published, versioned Gitea
+// module (gitea.iliadenisov.ru/developer/scrabble-solver, pinned in backend/go.mod). It
+// is fetched directly from Gitea — set GOPRIVATE=gitea.iliadenisov.ru/* so go skips the
+// public proxy/checksum DB. For local solver co-development, temporarily add:
+//   replace gitea.iliadenisov.ru/developer/scrabble-solver => ../scrabble-solver
 
 // scrabble/pkg holds the shared wire contracts (push proto + FlatBuffers edge
 // payloads) imported by both backend and gateway. Its module path has no dot, so