Per-site configurable cookie categories (#3)

* feat: per-site configurable cookie categories

Operators can now choose which cookie categories the banner displays
for a given site — useful for sites that genuinely don't use
e.g. marketing cookies and shouldn't be forced to show the toggle.

**Backend**

* New ``enabled_categories`` JSONB column on ``site_configs``,
  ``site_group_configs``, and ``org_configs`` (migration 0003).
  NULL at a level means "inherit"; an explicit list overrides.
* ``config_resolver`` merges ``enabled_categories`` through the
  existing cascade (system → org → group → site) and normalises
  the result via ``_normalise_enabled_categories``:
  - Unknown slugs stripped.
  - ``necessary`` is forced in regardless of the operator's input
    — it's never optional.
  - Empty / invalid values fall back to the full five-category
    default so a cleared field doesn't silently hide the banner.
  - Output is returned in canonical display order so insertion
    order from the cascade doesn't leak into the UI.
* ``build_public_config`` surfaces ``enabled_categories`` to the
  banner-facing public config endpoint.
* Schemas for site/group/org config create + update + response all
  include the new field.

**Banner**

* ``apps/banner/src/banner.ts`` replaces the hard-coded
  ``ALL_CATEGORIES`` / ``NON_ESSENTIAL`` constants with a runtime
  ``resolveEnabledCategories(config)`` helper. ``renderCategories``
  takes the enabled list and only renders toggles for those
  categories; ``nonEssentialFor(enabled)`` derives the user-toggleable
  subset. Falls back to all five when the field is missing in the
  config payload so older banner bundles against newer APIs (and
  vice versa) don't break.
* ``SiteConfig`` type in ``apps/banner/src/types.ts`` has
  ``enabled_categories?: CategorySlug[]`` to match.

**Admin UI**

* New ``SiteCategoriesTab`` component — five checkboxes, ``necessary``
  locked on, with "Reset to inherited" to clear the site override.
  Wired in as a new core tab on ``SiteDetailPage`` between
  Configuration and Cookies.
* ``SiteConfig`` type in ``types/api.ts`` declares ``enabled_categories``
  and a new ``ALL_COOKIE_CATEGORIES`` constant exposing label/description
  metadata shared between the tab component and any future display of
  the list.

**Semantics of a disabled category**

When the operator unticks e.g. ``marketing`` for a site:

* The toggle is not rendered in the banner.
* A visitor can never grant consent for ``marketing``.
* Any cookie or script that classifies into ``marketing`` stays
  blocked permanently by the auto-blocker.

That's the correct behaviour for sites that genuinely don't use a
category: declare it, hide it from the visitor, have the blocker
enforce it.

**Tests**

* ``test_config_resolver.py`` — 13 new cases covering the full
  cascade, ``necessary`` forcing, unknown-slug stripping, empty /
  non-list values, canonical display order, and the public-config
  surface. 37 passed total.
* ``test_SiteCategoriesTab.test.tsx`` — renders all five, locks
  ``necessary``, pre-fills from an override, saves the explicit
  list, and resets to inherited by sending NULL. 6 cases.
* Full API suite (610) and admin-ui suite (139) both green;
  banner bundle builds cleanly with 363 tests passing.

* style: ruff format config_resolver.py

apps/admin-ui/src/test/SiteConfigTab.test.tsx

@@ -41,6 +41,7 @@ const BASE_CONFIG: SiteConfig = {
   scan_enabled: true,
   scan_frequency_hours: 168,
   scan_max_pages: 50,
+  enabled_categories: null,
   created_at: '2025-01-01T00:00:00Z',
   updated_at: '2025-01-01T00:00:00Z',
 };