fix(docker): fail fast on CPUs that lack SSE4.2 support (#118)

On old hardware (e.g. AMD Turion/Phenom, Intel Core 2), sharp's musl binary
crashes with SIGILL during NestJS module init. Combined with bufferLogs: true
and restart: unless-stopped, this produces a silent infinite restart loop -
only the migration success message is ever visible.

Add check_cpu_sse42 to entrypoint.sh: greps /proc/cpuinfo for sse4_2, exits
with a clear error if absent, no-op when /proc/cpuinfo is unavailable. Also
add a System Requirements section to README documenting the SSE4.2 minimum.

Closes #116

README.md

@@ -72,6 +72,18 @@ Note: The demo includes a sample library of public domain books. Some features a
 
 ---
 
+## System Requirements
+
+| Component    | Minimum                                                                         |
+| ------------ | ------------------------------------------------------------------------------- |
+| CPU (x86-64) | SSE4.2 support required - Intel Core (Nehalem, 2008+) or AMD (Bulldozer, 2011+) |
+| RAM          | 1 GB (2 GB+ recommended)                                                        |
+| Docker       | 20.10+                                                                          |
+
+> **CPU note:** The image processing library used for book covers requires SSE4.2 instructions. CPUs that pre-date Intel Nehalem (2008) or AMD Bulldozer (2011) do not meet this requirement - including the entire Intel Core 2 series, AMD Phenom, and AMD Turion families. If your CPU is incompatible, the container will exit immediately with a clear error message rather than looping silently.
+
+---
+
 ## Quick Start
 
 ```bash