perf: build frontend once on runner instead of in Docker

- [x] Remove Docker compilation from server, scanner, migrate Dockerfiles
- [x] Use pre-built binaries injected by GoReleaser instead
- [x] Delete separate gateway container and merge into frontend
- [x] Update .goreleaser.yaml to reference pre-built binaries
- [x] Simplify Kubernetes deployment to remove gateway service
- [x] Simplify docker-compose to remove gateway container
- [x] Add backend proxy configuration to frontend

Dockerfile.server

@@ -1,50 +1,10 @@
-# Application Engine - GoHoarder Server
-# Multi-stage build to compile with CGO support
+# Application Engine - Main GoHoarder Server
+# This Dockerfile expects a PRE-BUILT binary from GoReleaser (no compilation)
+# GoReleaser injects the platform-specific binary automatically
 
-# Build stage
-# Let buildx handle platform automatically
-FROM golang:1.25-alpine AS builder
-
-# Get build platform from buildx
-ARG TARGETPLATFORM
-ARG TARGETOS
-ARG TARGETARCH
-
-# Install build dependencies for CGO
-RUN apk add --no-cache \
-    gcc \
-    g++ \
-    musl-dev \
-    sqlite-dev \
-    git
-
-WORKDIR /build
-
-# Copy go mod files
-COPY go.mod go.sum ./
-RUN go mod download
-
-# Copy source code
-COPY . .
-
-# Build with CGO enabled
-ARG TARGETOS
-ARG TARGETARCH
-ARG VERSION=dev
-ARG GIT_COMMIT=unknown
-ARG BUILD_TIME=unknown
-
-RUN CGO_ENABLED=1 GOOS=$TARGETOS GOARCH=$TARGETARCH \
-    go build -ldflags="-s -w \
-    -X github.com/lukaszraczylo/gohoarder/internal/version.Version=${VERSION} \
-    -X github.com/lukaszraczylo/gohoarder/internal/version.GitCommit=${GIT_COMMIT} \
-    -X github.com/lukaszraczylo/gohoarder/internal/version.BuildTime=${BUILD_TIME}" \
-    -o gohoarder ./cmd/gohoarder
-
-# Runtime stage
 FROM alpine:latest
 
-# Install runtime dependencies (including CGO/SQLite dependencies)
+# Install runtime dependencies (CGO/SQLite requires these)
 RUN apk add --no-cache \
     ca-certificates \
     tzdata \
@@ -66,8 +26,9 @@ RUN mkdir -p /var/cache/gohoarder \
     chmod -R 750 /var/cache/gohoarder \
                  /var/lib/gohoarder
 
-# Copy binary from builder
-COPY --from=builder /build/gohoarder /usr/local/bin/gohoarder
+# Copy pre-built binary from GoReleaser
+# GoReleaser will automatically inject the correct binary for the target platform
+COPY gohoarder /usr/local/bin/gohoarder
 RUN chmod +x /usr/local/bin/gohoarder
 
 # Copy example config