[Phase 3] Implement StorageBackend interface/trait for pluggable backends

js/.changeset/storage-backend-interface.md

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+---
+'links-queue-js': minor
+---
+
+Add pluggable StorageBackend interface for switching between storage backends via configuration
+
+- Add `StorageBackend` interface with lifecycle, CRUD, batch, and metadata operations
+- Add `BackendCapabilities` and `BackendStats` types for backend introspection
+- Add `MemoryBackendAdapter` wrapping `MemoryLinkStore` with `StorageBackend` interface
+- Add `BackendRegistry` for registering and creating backends by configuration
+- Add comprehensive tests for backend registry and adapter

js/src/backends/registry.d.ts

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+/**
+ * Backend registry type definitions for links-queue.
+ */
+
+import type {
+  StorageBackend,
+  BackendOptions,
+  BackendConstructor,
+  BackendFactory,
+  BackendCapabilities,
+  BackendStats,
+  MemoryBackendOptions,
+} from './types.ts';
+import type { Link, LinkId, LinkPattern } from '../types.ts';
+
+/**
+ * Memory backend adapter that wraps MemoryLinkStore with StorageBackend interface.
+ */
+export declare class MemoryBackendAdapter implements StorageBackend {
+  constructor(options?: MemoryBackendOptions);
+
+  connect(): Promise<void>;
+  disconnect(): Promise<void>;
+  isConnected(): boolean;
+
+  save(link: Link): Promise<LinkId>;
+  load(id: LinkId): Promise<Link | null>;
+  delete(id: LinkId): Promise<boolean>;
+  query(pattern: LinkPattern): Promise<Link[]>;
+
+  saveBatch(links: readonly Link[]): Promise<LinkId[]>;
+  deleteBatch(ids: readonly LinkId[]): Promise<boolean[]>;
+
+  getCapabilities(): BackendCapabilities;
+  getStats(): BackendStats;
+
+  clear(): Promise<void>;
+}
+
+/**
+ * Backend registry interface.
+ */
+export interface BackendRegistryInterface {
+  /**
+   * Registers a backend implementation.
+   */
+  register(name: string, backend: BackendConstructor | BackendFactory): void;
+
+  /**
+   * Unregisters a backend implementation.
+   */
+  unregister(name: string): boolean;
+
+  /**
+   * Checks if a backend is registered.
+   */
+  has(name: string): boolean;
+
+  /**
+   * Creates a backend instance from configuration.
+   */
+  create(config: BackendOptions): StorageBackend;
+
+  /**
+   * Lists all registered backend types.
+   */
+  listTypes(): string[];
+
+  /**
+   * Resets the registry to default state.
+   */
+  reset(): void;
+}
+
+/**
+ * Singleton backend registry instance.
+ */
+export declare const BackendRegistry: BackendRegistryInterface;