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/migrations/20260414000002_missing_fields.go

@@ -0,0 +1,71 @@
+// Migration 2 — add fields present in code but absent from pb-init-v3.sh.
+//
+// Discovered by auditing every PocketBase field access in the Go backend
+// and SvelteKit UI against the collection definitions in pb-init-v3.sh:
+//
+//   books.rating              (number) — written by WriteMetadata but never defined.
+//   app_users.notify_new_chapters_push (bool) — used in UI push-notification opt-in.
+//   book_comments.chapter     (number) — used to scope comments to a chapter (0 = book-level).
+//
+// The check for field existence makes this migration safe to re-apply on
+// a fresh install where migration 1 already created the collections without
+// these fields.
+package migrations
+
+import (
+	"github.com/pocketbase/pocketbase/core"
+	m "github.com/pocketbase/pocketbase/migrations"
+)
+
+func init() {
+	m.Register(func(app core.App) error {
+		type addition struct {
+			collection string
+			field      core.Field
+		}
+		additions := []addition{
+			{"books", &core.NumberField{Name: "rating"}},
+			{"app_users", &core.BoolField{Name: "notify_new_chapters_push"}},
+			{"book_comments", &core.NumberField{Name: "chapter"}},
+		}
+		for _, a := range additions {
+			coll, err := app.FindCollectionByNameOrId(a.collection)
+			if err != nil {
+				return err
+			}
+			if coll.Fields.GetByName(a.field.GetName()) != nil {
+				continue // already present — idempotent
+			}
+			coll.Fields.Add(a.field)
+			if err := app.Save(coll); err != nil {
+				return err
+			}
+		}
+		return nil
+	}, func(app core.App) error {
+		type removal struct {
+			collection string
+			field      string
+		}
+		removals := []removal{
+			{"books", "rating"},
+			{"app_users", "notify_new_chapters_push"},
+			{"book_comments", "chapter"},
+		}
+		for _, r := range removals {
+			coll, err := app.FindCollectionByNameOrId(r.collection)
+			if err != nil {
+				continue
+			}
+			f := coll.Fields.GetByName(r.field)
+			if f == nil {
+				continue
+			}
+			coll.Fields.RemoveById(f.GetId())
+			if err := app.Save(coll); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}