Replace Strapi CMS with PocketBase

Strapi + Postgres are gone in favor of PocketBase: a single Go binary
with embedded SQLite, built-in admin UI and per-collection API rules.
No content existed yet, so this is a clean swap with no data migration.

Collections and rules are defined as code in pocketbase/pb_migrations/
and applied automatically on first boot. Draft & Publish has no native
PocketBase equivalent, so it's reproduced with a nullable `publishedAt`
field enforced by listRule/viewRule, matching the old Strapi semantics.

Routing flips: PocketBase's admin UI and REST/file API are hardwired to
`/_/` and `/api/*` at the domain root (its own dashboard assets and API
calls reference those paths directly, so a stripped path prefix like
`/admin/*` would break them). `/api` is therefore reserved for
PocketBase now, and the frontend's Nitro endpoints move to `/content/*`
(frontend/server/routes/content/, not server/api/). A `/admin` vanity
route in Nitro (not Caddy) redirects to `/_/`, so it works the same in
dev, where Caddy isn't part of the stack, and in production.

frontend/server/utils/strapi.ts becomes pocketbase.ts; queries.ts is
rewritten for PocketBase's filter/sort/fields/expand query syntax.
StrapiImage becomes MediaImage (no width/height — PocketBase file
fields don't store dimensions, and the cover images already reserve
their aspect ratio via CSS, so this is not a regression).

docs/*.md, CLAUDE.md and README.md are updated in the same commit.

frontend/app/pages/category/[slug].vue

@@ -9,13 +9,13 @@ const page = computed(() => {
   return Number.isInteger(value) && value > 0 ? value : 1
 })
 
-const { data: category } = await useFetch<Category>(`/api/categories/${slug}`)
+const { data: category } = await useFetch<Category>(`/content/categories/${slug}`)
 
 if (!category.value) {
   throw createError({ statusCode: 404, statusMessage: 'Category not found', fatal: true })
 }
 
-const { data, error } = await useFetch<Paginated<ArticleSummary>>('/api/articles', {
+const { data, error } = await useFetch<Paginated<ArticleSummary>>('/content/articles', {
   query: { page, category: slug },
 })