Adjust Size model and create SizeRangeSize for sizes_sizegroup join table

[Copilot]

The DB migration in dropapp#821 removes sizes.sizegroup_id and sizes.seq, replacing the direct FK with a new sizes_sizegroup cross-reference table. This PR updates the BE models and all query code to match.

Model changes

• Size: removed seq and size_range (sizegroup_id FK) fields
• SizeRangeSize (new): maps sizes_sizegroup with size_id → sizes and sizegroup_id → sizegroup, using CompositeKey("size", "size_range")
• models/__init__.py: registers SizeRangeSize in MODELS

Query updates

All code that previously joined Size to SizeRange directly now routes through SizeRangeSize:

# Before: direct join on shared column (column no longer exists)
Product.select(...).left_outer_join(Size, on=(Product.size_range == Size.size_range))

# After: two-step lookup via join table
SizeRangeSize.select(SizeRangeSize, Size).join(Size)
# or for traversal queries:
Size.select(Size.id).join(SizeRangeSize).join(SizeRange).join(Product)

Affected files: loaders.py (SizesForSizeRangeLoader), warehouse/box/crud.py (bulk create), mobile_distribution/crud.py, cron/data_faking.py.

Test data

• Removed size_range from size fixtures
• Added size_range_size.py test data module; inserted into _NAMES ordering after size

[pylipp]

https://trello.com/c/Ld7cSa0s

[sentry]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.77%. Comparing base (4ab7e36) to head (7677001).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2679   +/-   ##
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  Coverage   76.77%   76.77%           
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  Files         300      300           
  Lines       22182    22182           
  Branches     2232     2232           
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  Hits        17030    17030           
  Misses       5104     5104           
  Partials       48       48           

Flag	Coverage Δ
frontend	69.10% <ø> (ø)
sharedComponents	68.72% <ø> (ø)

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