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>

backend/cmd/pocketbase/main.go

@@ -0,0 +1,47 @@
+// Command pocketbase is a thin wrapper that runs PocketBase as a Go framework
+// with version-controlled Go migrations.
+//
+// On every `serve`, PocketBase automatically applies any pending migrations from
+// the migrations/ package before accepting traffic.
+//
+// Usage (Docker):
+//
+//	./pocketbase serve --dir /pb_data --http 0.0.0.0:8090
+//
+// Migration workflow:
+//
+//	# Generate a timestamped stub:
+//	go run ./cmd/pocketbase migrate create "description"
+//	# Apply manually (also runs automatically on serve):
+//	go run ./cmd/pocketbase migrate up
+//	# Revert last migration:
+//	go run ./cmd/pocketbase migrate down 1
+//	# After migrating an existing install, mark existing schema as done:
+//	go run ./cmd/pocketbase migrate history-sync
+package main
+
+import (
+	"log"
+
+	"github.com/pocketbase/pocketbase"
+	"github.com/pocketbase/pocketbase/plugins/migratecmd"
+
+	// Register all migrations via init().
+	_ "github.com/libnovel/backend/migrations"
+)
+
+func main() {
+	app := pocketbase.New()
+
+	// Register the migrate sub-command.
+	// Automigrate: false — migrations are written by hand, never auto-generated
+	// from Admin UI changes. Pending migrations still apply automatically on
+	// every `serve` regardless of this flag.
+	migratecmd.MustRegister(app, app.RootCmd, migratecmd.Config{
+		Automigrate: false,
+	})
+
+	if err := app.Start(); err != nil {
+		log.Fatal(err)
+	}
+}