chore: remove migration scripts and endpoint after successful migration

Migration completed successfully. Removing one-time migration files:
- Migration endpoint (api/admin/migrate-image-hashes)
- Migration shell script
- Migration documentation

Core image hashing functionality remains in place for all future uploads.

scripts/migrate-image-hashes.sh

@@ -1,62 +0,0 @@
-#!/bin/bash
-
-# Image Hash Migration Script
-# This script triggers the image hash migration endpoint in production
-# It will:
-# 1. Find all images without hashes (shortname.webp)
-# 2. Generate content hashes for them
-# 3. Copy them to shortname.{hash}.webp (keeps originals)
-# 4. Update the database
-
-set -e
-
-echo "======================================"
-echo "Image Hash Migration Script"
-echo "======================================"
-echo ""
-echo "This will migrate all existing images to use content-based hashes."
-echo "Images will be copied from 'shortname.webp' to 'shortname.{hash}.webp'"
-echo "Original unhashed files will be kept for graceful degradation."
-echo ""
-echo "⚠️  WARNING: This operation will modify the database and create new files!"
-echo ""
-
-# Check for admin token
-if [ -z "$ADMIN_SECRET_TOKEN" ]; then
-    echo "Error: ADMIN_SECRET_TOKEN environment variable not set."
-    echo ""
-    echo "Please set it first:"
-    echo "  export ADMIN_SECRET_TOKEN='your-secret-token'"
-    echo ""
-    echo "Or source your .env file:"
-    echo "  source .env"
-    exit 1
-fi
-
-read -p "Are you sure you want to continue? (yes/no): " confirm
-
-if [ "$confirm" != "yes" ]; then
-    echo "Migration cancelled."
-    exit 0
-fi
-
-echo ""
-echo "Starting migration..."
-echo ""
-
-# Get the production URL (modify this to your production URL)
-PROD_URL="${PROD_URL:-https://bocken.org}"
-
-# Make the API call with admin token
-response=$(curl -s -X POST \
-    -H "Content-Type: application/json" \
-    -d "{\"confirm\": \"MIGRATE_IMAGES\", \"adminToken\": \"$ADMIN_SECRET_TOKEN\"}" \
-    "${PROD_URL}/api/admin/migrate-image-hashes")
-
-# Pretty print the response
-echo "$response" | jq '.' 2>/dev/null || echo "$response"
-
-echo ""
-echo "======================================"
-echo "Migration complete!"
-echo "======================================"