RustBox 0.1.2 release manifest

Storage model:
- database_backend = sqlite | mysql | pgsql selects the full application database
- session_backend = database stores sessions in the same database
- session_backend = redis uses Redis or Redis Sentinel for sessions

Database baselines:
- migrations/sqlite/schema.sql: complete SQLite schema, baseline version 46
- migrations/mysql/schema.sql: complete MySQL schema, baseline version 46
- migrations/pgsql/schema.sql: complete PostgreSQL schema, baseline version 46
- each schema includes global sessions and isolated account-data tables
- historical SQLite V001-V046 files are intentionally squashed out of this release
- next migration number for every SQL engine: V047

Cluster storage rules (0.1.2):
- SQL mailbox metadata/body cache is shared by the configured MySQL/PostgreSQL backend
- data_dir must be shared between nodes for filesystem-backed draft attachments
- cluster_shared_data_dir=true is required as an explicit deployment acknowledgement
- built-in Tantivy must be disabled in cluster mode (search_index_backend=disabled)
- OpenPGP must be disabled in cluster mode (pgp_enabled=false)
- legacy db.sqlite files remain migration-only; they are not the active database on MySQL/PostgreSQL

Compatibility:
- legacy RustBox 0.1.0 per-user SQLite databases at user_version 46 are imported on first access
- imported source DBs are retained as db.sqlite.pre-0.1.1.bak
- older legacy schemas must first be upgraded with RustBox 0.1.0

Tests/source cleanup:
- tests/fixtures/account-session-expired.json is retained because Rust tests include it
- stale tests/*.py and tests/theme_fixture.html are not part of the release tree

Oxi tracking:
- UPSTREAM_BASE stores the consumed Oxi revision
- backend source changes are mapped into the flat RustBox tree
- upstream backend/migrations and backend/tests are review-only and never auto-imported
- frontend changes are reviewed and manually ported to src/web/
- scripts/finish-upstream.sh requires explicit acknowledgement for changed RustBox-owned upstream areas before advancing UPSTREAM_BASE