feat: Replace JSON memory with pgvector semantic search (MAT-11)

Add memory-service (FastAPI + pgvector) for semantic memory storage.
Bot now queries relevant memories per conversation instead of dumping all 50.
Includes migration script for existing JSON files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

memory-service/Dockerfile

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+FROM python:3.11-slim
+WORKDIR /app
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+COPY main.py .
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8090"]