fix: remove hardcoded platform args causing wrong architecture builds

CRITICAL FIX: arm64 images were getting amd64 binaries causing
"exec format error" when running on ARM platforms.

Root cause:
- Dockerfiles had ARG TARGETOS=linux and ARG TARGETARCH=amd64
- FROM --platform=$TARGETOS/$TARGETARCH used these defaults
- buildx couldn't override them, so all builds used amd64

Solution:
- Removed --platform from FROM statement
- Let buildx automatically handle platform selection
- Declared ARGs AFTER FROM to receive buildx values
- buildx automatically sets TARGETPLATFORM, TARGETOS, TARGETARCH

Now each platform builds native binaries:
- linux/amd64 → amd64 binary
- linux/arm64 → arm64 binary

This fixes: exec /usr/local/bin/migrate: exec format error

Dockerfile.server

@@ -1,10 +1,14 @@
 # Application Engine - GoHoarder Server
 # Multi-stage build to compile with CGO support
-ARG TARGETOS=linux
-ARG TARGETARCH=amd64
 
 # Build stage
-FROM --platform=$TARGETOS/$TARGETARCH golang:1.25-alpine AS builder
+# 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 \