refactor: extract shared utilities for duplicated code patterns

[devin-ai-integration]

Summary

Refactors three duplicated patterns into shared utilities. Behavior is unchanged — all 7 existing tests pass and API serialization (category_ru, emoji, discount) is verified identical.

1. models.py — duplicated computed fields. Box and Order each defined byte-identical category_ru/emoji computed properties. Hoisted into a shared base both now inherit:

class CategoryLabeled(BaseModel):
    category: BoxCategory
    @computed_field @property
    def category_ru(self) -> str: return CATEGORY_RU.get(self.category, self.category)
    @computed_field @property
    def emoji(self) -> str: return CATEGORY_EMOJI.get(self.category, _DEFAULT_EMOJI)

class Box(CategoryLabeled): ...   # keeps its own `discount`
class Order(CategoryLabeled): ...

2. db.py — repeated SQL / row-mapping. Both _box_from_row and _order_from_row re-derived partner fields with the same p["..."] if p else <default> fallbacks; the case-insensitive code lookup was duplicated across order_by_code/redeem; and SELECT * FROM {orders,boxes} WHERE id=? was repeated in create/redeem/refund/box paths. Extracted helpers:

_partner_fields(c, pid)      # -> {partner_name, district, address, rating} with fallbacks
_order_row_by_code(c, code)  # SELECT ... WHERE code=? (code.strip().upper())
_order_row_by_id(c, id)
_box_row_by_id(c, id)

3. app/static/index.html — repeated fetch boilerplate. The let x=[];try{x=await get(url)}catch(e){} idiom appeared in every loader. Replaced with one helper:

const getSafe=(u,fallback=[])=>get(u).catch(()=>fallback);

used by loadStore, loadPartner, loadPartnerData, loadAdmin.

No functional changes; net -47/+59 lines across 3 files.

Link to Devin session: https://app.devin.ai/sessions/30bd78e5e39c432abf0292dcecc4d4eb
Requested by: @wpalish

[devin-ai-integration]

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