refactor(docker): split requirements by role, slim enterprise bundle by default

Split requirements.txt into role-specific files:
  - requirements-backend.txt — FastAPI API runtime
  - requirements-agent.txt — Gradio/LangGraph agent (no embeddings)
  - requirements-embeddings.txt — opt-in sentence-transformers

Default enterprise bundle (./build.sh bundle) builds slim agent without
sentence-transformers/torch (~500 MB saved). Embeddings opt-in via
./build.sh bundle:embeddings <tag>. Embedding router degrades gracefully
when package is absent (ImportError catch in _embedding_router.py).

Updated Dockerfiles:
  - backend.Dockerfile → requirements-backend.txt
  - Dockerfile.agent → requirements-agent.txt + WITH_EMBEDDINGS build arg
  - all-in-one.Dockerfile → requirements-backend.txt

Added embeddings variant to build.sh with full manifest/digest output.
Hardened .dockerignore (cache, report, model, archive patterns).

Other branch work: align git route mocks with async services, remove
legacy agent test files superseded by contract tests.

docker/Dockerfile.agent

@@ -20,10 +20,18 @@ WORKDIR /app
 RUN apt-get update && apt-get install -y --no-install-recommends \
     libgl1 libglib2.0-0 && rm -rf /var/lib/apt/lists/*
 
-# Python dependencies — используем backend/requirements.txt (содержит httpx, gradio,
-# langgraph, langchain-openai, pdfplumber и все транзитивные зависимости)
-COPY backend/requirements.txt /app/backend/requirements.txt
-RUN pip install --no-cache-dir -r /app/backend/requirements.txt
+# Python dependencies — agent runtime (gradio, langgraph, httpx, pdfplumber, ...)
+# Без sentence-transformers — embedding router безопасно отключается при ImportError.
+ARG WITH_EMBEDDINGS=false
+
+COPY backend/requirements-agent.txt /app/backend/requirements-agent.txt
+RUN pip install --no-cache-dir -r /app/backend/requirements-agent.txt
+
+# Optional: semantic embedding routing (большие ML-зависимости: ~500 MB+)
+COPY backend/requirements-embeddings.txt /app/backend/requirements-embeddings.txt
+RUN if [ "$WITH_EMBEDDINGS" = "true" ]; then \
+        pip install --no-cache-dir -r /app/backend/requirements-embeddings.txt; \
+    fi
 
 # ── Минимальная копия backend/src/ ──────────────────────────────
 # Только то, что реально импортирует агент (трассировка 2026-06-14):