fix: use single backslash for Dockerfile line continuations

The compile_dockerfile function used \\\\ in Python string literals,
which produces \ (two backslashes) in the Dockerfile output. Docker's
legacy builder requires a single backslash \ for line continuation.

This caused 'unknown instruction: curl' because Docker saw the first as the continuation and the second \ as a literal character before the
newline, breaking the RUN command parsing.

Fix: change all \\ to \ in Python string literals within
compile_dockerfile, producing the correct single-backslash continuation.

Verified with hex dump from container logs:
- Before: line ended with 5c5c (two backslashes)
- After: line ends with 5c (one backslash)

apps/api/src/services/manifest_compiler.py

@@ -117,10 +117,10 @@ def compile_dockerfile(manifest: dict) -> str:
     # System packages (apt)
     apt_packages = manifest.get("packages", {}).get("apt", [])
     if apt_packages:
-        lines.append("RUN apt-get update && apt-get install -y \\\\")
+        lines.append("RUN apt-get update && apt-get install -y \\")
         for pkg in apt_packages[:-1]:
-            lines.append(f"    {pkg} \\\\")
-        lines.append(f"    {apt_packages[-1]} \\\\")
+            lines.append(f"    {pkg} \\")
+        lines.append(f"    {apt_packages[-1]} \\")
         lines.append("    && rm -rf /var/lib/apt/lists/*")
         lines.append("")
 
@@ -129,9 +129,9 @@ def compile_dockerfile(manifest: dict) -> str:
     if node:
         version = node.get("version", "20")
         lines.append(
-            f"RUN curl -fsSL https://deb.nodesource.com/setup_{version}.x | bash - && \\\\"
+            f"RUN curl -fsSL https://deb.nodesource.com/setup_{version}.x | bash - && \\"
         )
-        lines.append("    apt-get install -y nodejs && \\\\")
+        lines.append("    apt-get install -y nodejs && \\")
         lines.append("    rm -rf /var/lib/apt/lists/*")
         lines.append("")
 
@@ -157,7 +157,7 @@ def compile_dockerfile(manifest: dict) -> str:
         gid = user["gid"]
         create_home = "-m " if user.get("create_home", True) else ""
         shell = user.get("shell", "/bin/bash")
-        lines.append(f"RUN groupadd -g {gid} {name} && \\\\")
+        lines.append(f"RUN groupadd -g {gid} {name} && \\")
         lines.append(f"    useradd -u {uid} -g {gid} {create_home}-s {shell} {name}")
         lines.append("")