feat!: introduce storage abstraction layer

[paullla]

Q	A
Bug fix?	yes (removed a method that threw TypeError on every hit, see below)
New feature?	yes
BC breaks?	yes
Deprecations?	no
Issues	-
License	MIT

Problem

Loggastic is hard-wired to Elasticsearch: the data processor, both data providers, the populate handler and the console commands all inject ElasticsearchServiceInterface, ElasticsearchContextFactoryInterface or ElasticsearchIndexFactoryInterface directly. There is no way to store activity logs in another backend without forking, and #32 (bridge interfaces) only made the existing Elasticsearch services decoratable. #38 prepared the modern bundle layout so this abstraction could land on top of it.

Changes

• New Locastic\Loggastic\Storage namespace with three storage-agnostic interfaces: ActivityLogStorageInterface (write/read activity logs), CurrentDataTrackerStorageInterface (save/update/bulk-save/find trackers) and StorageInitializerInterface (create or recreate the underlying storage per loggable class, idempotent initialize* methods return whether the storage was created)
• Elasticsearch implementations in Bridge\Elasticsearch\Storage (ElasticsearchActivityLogStorage, ElasticsearchCurrentDataTrackerStorage, ElasticsearchStorageInitializer), aliased to the interfaces, so existing apps need no configuration changes. Custom backends implement the three interfaces and override the aliases (documented in the README)
• Breaking: ActivityLogProcessor, ActivityLogProvider, CurrentDataTrackerProvider, PopulateCurrentDataTrackersHandler and both console commands now depend on the storage interfaces; their constructor signatures changed
• Breaking: ActivityLogProviderInterface::getActivityLogsByIndexAndId() (from Add limit and offset to ActivityLogProvider #24) moved to ElasticsearchActivityLogStorage::findByIndexAndObjectId(): an index-name-based read is inherently Elasticsearch-specific and does not belong on the storage-agnostic provider
• Breaking: ElasticNormalizationContextTrait moved to Serializer\Traits\NormalizationContextTrait; it only forces the \DateTime::ATOM date format and was the last Bridge\Elasticsearch reference in the logger and update handler
• Removed ActivityLogProvider::getCurrentDataTrackerByClassAndId(): it declared an ?array return type while the denormalizer returns a CurrentDataTracker object, so any call that found a tracker threw a TypeError (CurrentDataTrackerProviderInterface provides the working equivalent)
• Removed the unused IndexNotFoundException; CreateLoggableIndexesCommand no longer catches the Elasticsearch client exception itself, the "already exists" handling lives in the initializer
• CHANGELOG, UPGRADE-2.0.md and README updated (custom storage backend how-to); PHPStan baseline shrank by the entries for removed methods, no new entries added

Verification

• Full PHPUnit suite (24 tests, 53 assertions) passes against Elasticsearch 9.0
• locastic:activity-logs:create-loggable-indexes exercised twice end-to-end through the test kernel: first run creates all indexes, second run prints "Index already exists, skipping." for each and exits 0
• PHPStan reports no errors; PHP-CS-Fixer reports 0 fixable files; composer validate --strict passes