Populate and use subcategory field on community gear

[thomasbecker]

Problem

The community_gear table has a subcategory column that was designed in
the original product spec but is never populated or displayed. This means
all tents, tarps, hammocks, and bivvies are lumped under "Shelter" — and
all quilts, sleeping bags, and pads are lumped under "Sleep System".

This makes it hard to browse gear by specific type (e.g., "show me all
quilts sorted by weight").

Proposal

1. Define known subcategory enums per category

Use a typed enum for common subcategories, with free-text fallback for
anything that doesn't fit:

Category	Known Subcategories
Shelter	tent, tarp, hammock, bivy
Sleep System	sleeping bag, quilt, sleeping pad
Pack	backpack, daypack, fanny pack
Bags & Containers	stuff sack, dry bag, packing cube, pannier
Cook Kit	stove, pot, utensil, mug
Water	bottle, filter, bladder
Clothing	jacket, base layer, rain shell, pants, gloves, hat, socks, shoes
Electronics	headlamp, battery, solar, gps
Hygiene	(leave free-text)
Navigation	(leave free-text)
Emergency	(leave free-text)
Misc	(leave free-text)

Items that don't match any known subcategory keep the free-text field.

2. Populate existing gear

• Backfill subcategories for existing community gear items (AI-assisted
  or rule-based by name matching)
• Update smart gear entry to extract subcategory during AI enrichment

3. Expose in UI

• Display subcategory on gear detail pages
• Use as a filter dimension in the community gear list
• Foundation for the Filterable Gear Catalog feature (M47)

Context

This is a prerequisite for M47 (Filterable Gear Catalog), which will add
category landing pages with filtering and sorting by weight.