Add plants:import_categories for the ECHOcommunity plant migration

[npflood]

Phase 1 of the plant-data ownership migration. Companion to #117 and #118, but
independent of both — merge in any order.

Creates missing plant categories and syncs membership from ECHOcommunity.

bin/rails plants:import_categories[tmp/categories.json]              # dry run
APPLY=true bin/rails plants:import_categories[tmp/categories.json]   # write

Categories share the UUID convention: ECHOcommunity's Resource id for a category
is this application's categories.id. That holds for all 14 categories the
two systems have in common, so no mapping table is needed.

Membership is not cosmetic. The GMCC selector gates on category membership,
so a plant that loses its links disappears from that tool entirely.

Behaviour worth reviewing

• Additive. Existing links are kept and missing ones added; removing a link
  is an editorial act, not a migration one.
• Categories are built then saved, not created — name is a required
  translated attribute, so create! validates before apply_translations has a
  chance to set it. That was a real failure, not a hypothetical one.
• A dry run counts links for categories it is about to create. Otherwise it
  under-reports by the entire membership of every new category — 179 links for
  Bamboo alone — making the dry run look far smaller than the actual change.

Rehearsed against staging

Created the missing Bamboo category (182 ECHOcommunity plants; it postdates
the 2020 export) and 179 links, bringing categories to parity at 15 and links to
815.

The 8-link difference against ECHOcommunity's 823 is fully explained: 11 of
its rows point at plants that do not exist — orphans in a join table that has
no primary key, no index and no foreign keys. They are unmigratable by
definition, so 812 is the correct number to carry across.

• 6 new specs
• Full suite: 2,310 examples, 0 failures
• Rubocop clean

[npflood]

Rebased onto master to clear a conflict in .rubocop.yml.

#117 and this PR each add a line to the same Metrics/BlockLength exclude list — plant_import.rake and plant_categories.rake — so once #117 merged, git saw competing edits on adjacent lines. Both belong; the resolution keeps them.

Re-verified against merged master: 2,353 examples, 0 failures, whole-repo rubocop clean across 744 files.