feat: add immutable corpus version creation

[jemsbhai]

Outcome

Adds append-only immutable corpus version creation for existing corpora through manual input or bounded UTF-8 TXT, CSV, and JSON imports.

What changed

• Adds server-owned v2+ lineage, monotonically increasing version numbers, historical reads/exports, and CorpusGen 0.1.7 provenance.
• Applies writer authorization, tenant-scoped hierarchy checks, PostgreSQL locking, duplicate-content conflicts, atomic retained-sentence quota accounting, and text-free audit evidence.
• Adds accessible project-workbench creation flows, viewer read-only controls, stale-selection protection, partial-success recovery, and focus management.
• Updates the OpenAPI snapshot, product/operations documentation, and mocked plus real-stack acceptance coverage.

Security and data impact

Corpus versions remain immutable. Parent selection and numbering are server-owned. Cross-tenant identifiers stay opaque, quota/version/audit writes share one transaction, and audit metadata never includes sentence text. No schema migration or dependency change is included.

Validation

• Backend: 1,498 passed, 80 explicit environment/platform skips, 0 failed; 94% coverage.
• Ruff formatting/lint and strict mypy: passed.
• OpenAPI exact-sync and backward-compatibility checks: passed.
• Web: 496 tests passed; Prettier, ESLint, typecheck, and production build passed.
• Browser: 77 passed across Chromium, Firefox, and WebKit; 4 explicit environment/platform skips.
• Opt-in live PostgreSQL/external-stack cases were unavailable locally; lock SQL, transaction rollback, client contracts, and mocked browser behavior are covered.

Rollback

Revert the merge commit. This change adds no database migration and leaves existing version-1 data/contracts intact.