feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/backend/Dockerfile

@@ -0,0 +1,42 @@
+# syntax=docker/dockerfile:1
+FROM golang:1.26.1-alpine AS builder
+WORKDIR /app
+
+# Download modules into the BuildKit cache so they survive across builds.
+# This layer is only invalidated when go.mod or go.sum changes.
+COPY go.mod go.sum ./
+RUN --mount=type=cache,target=/root/go/pkg/mod \
+    go mod download
+
+COPY . .
+
+ARG VERSION=dev
+ARG COMMIT=unknown
+
+# Build all three binaries in a single layer so the Go compiler can reuse
+# intermediate object files.  Both cache mounts are preserved between builds:
+#   /root/go/pkg/mod   — downloaded module source
+#   /root/.cache/go-build — compiled package objects (incremental recompile)
+RUN --mount=type=cache,target=/root/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 GOOS=linux go build \
+      -ldflags="-s -w -X main.version=${VERSION} -X main.commit=${COMMIT}" \
+      -o /out/backend ./cmd/backend && \
+    CGO_ENABLED=0 GOOS=linux go build \
+      -ldflags="-s -w -X main.version=${VERSION} -X main.commit=${COMMIT}" \
+      -o /out/runner ./cmd/runner && \
+    CGO_ENABLED=0 GOOS=linux go build \
+      -ldflags="-s -w" \
+      -o /out/healthcheck ./cmd/healthcheck
+
+# ── backend service ──────────────────────────────────────────────────────────
+FROM gcr.io/distroless/static:nonroot AS backend
+COPY --from=builder /out/healthcheck /healthcheck
+COPY --from=builder /out/backend /backend
+ENTRYPOINT ["/backend"]
+
+# ── runner service ───────────────────────────────────────────────────────────
+FROM gcr.io/distroless/static:nonroot AS runner
+COPY --from=builder /out/healthcheck /healthcheck
+COPY --from=builder /out/runner /runner
+ENTRYPOINT ["/runner"]