feat(phase1): add image storage utilities

- Create backend/services/image_storage.py with 4 core functions:
  - sanitize_filename(): remove unsafe chars, limit to 255 chars, convert to lowercase
  - get_unique_filename(): handle collisions with UUID suffix (format: {name}_{uuid8}_{variant}.jpg)
  - ensure_image_directories(): create /images/ root and category subdirs on startup
  - save_image(): save bytes to /images/{category}/{filename}, returns relative path
- Create comprehensive test suite (22 tests) covering all functionality
- Integrate ensure_image_directories() into FastAPI startup event
- Directory structure: /images/{category}/{filename}
- Collision handling: auto-suffix with UUID if filename exists
- All tests passing, pathlib.Path for safe operations

venv/lib/python3.12/site-packages/yaml/nodes.py

@@ -0,0 +1,49 @@
+
+class Node(object):
+    def __init__(self, tag, value, start_mark, end_mark):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+    def __repr__(self):
+        value = self.value
+        #if isinstance(value, list):
+        #    if len(value) == 0:
+        #        value = '<empty>'
+        #    elif len(value) == 1:
+        #        value = '<1 item>'
+        #    else:
+        #        value = '<%d items>' % len(value)
+        #else:
+        #    if len(value) > 75:
+        #        value = repr(value[:70]+u' ... ')
+        #    else:
+        #        value = repr(value)
+        value = repr(value)
+        return '%s(tag=%r, value=%s)' % (self.__class__.__name__, self.tag, value)
+
+class ScalarNode(Node):
+    id = 'scalar'
+    def __init__(self, tag, value,
+            start_mark=None, end_mark=None, style=None):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.style = style
+
+class CollectionNode(Node):
+    def __init__(self, tag, value,
+            start_mark=None, end_mark=None, flow_style=None):
+        self.tag = tag
+        self.value = value
+        self.start_mark = start_mark
+        self.end_mark = end_mark
+        self.flow_style = flow_style
+
+class SequenceNode(CollectionNode):
+    id = 'sequence'
+
+class MappingNode(CollectionNode):
+    id = 'mapping'
+