fix(entrypoint): convert CRLF to LF to fix Docker execution

exec ./entrypoint.sh: no such file or directory was caused by
Windows-style line endings making the shebang unresolvable.

Also add .gitattributes to enforce LF for shell scripts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,2 @@
+* text=auto
+*.sh text eol=lf

backend/entrypoint.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#\!/bin/bash
 set -e
 echo "[entrypoint] Running database seed..."
 python seed_db.py