feat: replace pb-init-v3.sh with PocketBase Go migrations

Adds a custom PocketBase binary (cmd/pocketbase) that embeds PocketBase as a
Go framework with version-controlled migrations, replacing the fragile 419-line
shell script. Migrations apply automatically on every `serve` startup.

Migration 1 (20260414000001): full baseline schema — all 21 collections, both
  indexes on chapters_idx, and initial superuser creation from env vars.
Migration 2 (20260414000002): adds three fields found in code but missing from
  the old script — books.rating, app_users.notify_new_chapters_push,
  book_comments.chapter.

docker-compose: pocketbase service now uses the custom kalekber/libnovel-pocketbase
image; pb-init one-shot container removed (migrations replace it entirely).

Existing installs: run `migrate history-sync` once to mark migration 1 as done,
then restart — migration 2 will apply the three previously missing fields.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

justfile

@@ -56,7 +56,7 @@ build-svc svc:
 
 # Push all custom images to Docker Hub (requires docker login)
 push:
-    {{doppler}} docker compose push backend runner ui caddy
+    {{doppler}} docker compose push backend runner ui caddy pocketbase
 
 # Build then push all custom images
 build-push: build push