Add shared core library and architecture plans (Phase 1)

Extract HA-independent logic from the integration into packages/core/
as a standalone Python library (immich-watcher-core). This is the first
phase of restructuring the project to support a standalone web app
alongside the existing HAOS integration.

Core library modules:
- models: SharedLinkInfo, AssetInfo, AlbumData, AlbumChange dataclasses
- immich_client: Async Immich API client (aiohttp, session-injected)
- change_detector: Pure function for album change detection
- asset_utils: Filtering, sorting, URL building utilities
- telegram/client: Full Telegram Bot API (text, photo, video, media groups)
- telegram/cache: File ID cache with pluggable storage backend
- telegram/media: Media size checks, URL extraction, group splitting
- notifications/queue: Persistent notification queue
- storage: StorageBackend protocol + JSON file implementation

All modules have zero Home Assistant imports. 50 unit tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/pyproject.toml

@@ -0,0 +1,26 @@
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"
+
+[project]
+name = "immich-watcher-core"
+version = "0.1.0"
+description = "Core library for Immich album change detection and notifications"
+requires-python = ">=3.12"
+dependencies = [
+    "aiohttp>=3.9",
+]
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=8.0",
+    "pytest-asyncio>=0.23",
+    "aioresponses>=0.7",
+]
+
+[tool.hatch.build.targets.wheel]
+packages = ["src/immich_watcher_core"]
+
+[tool.pytest.ini_options]
+asyncio_mode = "auto"
+testpaths = ["tests"]