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/shared/types/blog.ts

@@ -1,10 +1,8 @@
-/** Shape of the Strapi payloads, narrowed to what the site actually renders. */
+/** Shape of the PocketBase payloads, narrowed to what the site actually renders. */
 
-export interface StrapiImage {
+export interface MediaImage {
   url: string
-  alternativeText: string | null
-  width: number | null
-  height: number | null
+  alt: string | null
 }
 
 export interface Category {
@@ -17,7 +15,7 @@ export interface ArticleSummary {
   title: string
   slug: string
   publishedAt: string
-  cover: StrapiImage | null
+  cover: MediaImage | null
   category: Category | null
 }
 
@@ -26,7 +24,7 @@ export interface Article extends ArticleSummary {
   html: string
   /** Plain-text opening of the body, used as the meta description. */
   summary: string
-  /** Byline: the full name of the admin user who wrote the article. */
+  /** Byline: the article's authorName field, filled in manually by the editor. */
   author: string | null
 }