feat: add ShareLink domain model and storage mixin

[yilu331]

Summary

Adds the ShareLink domain model and storage layer (ShareLinkMixin) to the open-source package. This is the open-source half of moving share links from an auth-side (Reflexio-managed) database to the per-customer data storage, so that share link records live alongside the resources they reference.

Changes

• Domain model (models/api_schema/domain/entities.py) — new ShareLink Pydantic class with id: int, org_id, token, resource_type, resource_id, created_at, expires_at, created_by_email.
• Storage protocol (server/services/storage/storage_base/_share_links.py) — abstract ShareLinkMixin with 6 methods: create_share_link, get_share_link_by_token, get_share_link_by_resource, get_share_links, delete_share_link, delete_all_share_links. BaseStorage now inherits this mixin.
• SQLite implementation (server/services/storage/sqlite_storage/_share_links.py, _base.py) — SQLiteShareLinkMixin plus the share_links table DDL (auto-created at startup). Uses the shared _epoch_now() helper and self._execute/_fetchone/_fetchall helpers for thread-safe queries.
• Disk stub (server/services/storage/disk_storage/_share_links.py) — DiskShareLinkMixin that raises NotImplementedError for each method (disk storage does not support share links).

Storage instances are org-scoped

None of the new methods take an org_id parameter — the storage instance is already bound to a specific org. This matches the pattern used by every other mixin.

Test plan

• New contract tests in tests/server/services/storage/test_sqlite_share_links.py cover create / get-by-token / get-by-resource / list / delete / delete-all (12 tests, all passing)
• Full storage test sweep: 148 tests passing (no regressions)
• ruff check / ruff format --check clean
• pyright clean on all new files