Add skeleton: config, migrations, startup sweep, two listeners

Step 1 of the build order in docs/decisions.md. Boots, applies migrations
before serving, and serves a health check and an empty admin page.

- 001_init.sql is the full schema from docs/spec.md, including the check
  constraints and indexes the old app lacked
- The startup sweep fails submissions left mid-conversion by a restart; an
  in-process goroutine dies with the process and those rows would otherwise
  say converting forever
- Admin is Basic Auth from env on its own listener, fatal at startup when
  ADMIN_PASSWORD is unset

Dockerfile

@@ -0,0 +1,15 @@
+FROM golang:1.24-alpine AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -o /levyraati .
+
+FROM alpine:3.21
+# yt-dlp rots against YouTube, so it is installed unpinned at build time and updated by rebuilding.
+RUN apk add --no-cache ffmpeg python3 py3-pip ca-certificates \
+    && pip install --break-system-packages --no-cache-dir -U yt-dlp
+COPY --from=build /levyraati /usr/local/bin/levyraati
+ENV STORAGE_DIR=/storage
+EXPOSE 8080
+ENTRYPOINT ["levyraati"]