feat(infra,database): DO deploy + memory schema

infra: Dockerfile (backend from monorepo root) + DigitalOcean App Platform
spec with health check and secret env vars. database: MongoDB Atlas + Voyage
vector-memory schema and the continual-learning loop notes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

infra/Dockerfile

@@ -0,0 +1,26 @@
+# PodMan backend agent — built from the monorepo root.
+# Build:  docker build -f infra/Dockerfile -t podman-backend .
+FROM node:24-slim AS base
+ENV PNPM_HOME=/pnpm
+ENV PATH="$PNPM_HOME:$PATH"
+RUN corepack enable
+WORKDIR /app
+
+# Install workspace deps (cached on lockfile)
+FROM base AS deps
+COPY pnpm-lock.yaml pnpm-workspace.yaml package.json ./
+COPY shared/package.json shared/
+COPY backend/package.json backend/
+RUN pnpm install --frozen-lockfile
+
+# Build shared + backend
+FROM deps AS build
+COPY . .
+RUN pnpm --filter @podman/shared build && pnpm --filter @podman/backend build
+
+# Runtime
+FROM base AS runtime
+ENV NODE_ENV=production
+COPY --from=build /app /app
+EXPOSE 8787
+CMD ["node", "backend/dist/index.js"]