chore: release 0.12.0

[pratyush618]

Cuts version 0.12.0.

Summary

• Bumps pyproject.toml, py_src/taskito/__init__.py, and the four crate Cargo.toml files from 0.11.1 → 0.12.0.
• Promotes the Unreleased section in docs/changelog.md to 0.12.0 and adds every commit landed on master since 0.11.1.

Highlights of 0.12.0

Added — dashboard persistent settings (branding, links, integrations) with optimistic UI; taskito worker --pool prefork CLI flag; URL-state for dead-letter page + view.

Changed — Preact dashboard fully replaced by React + Vite + TanStack Router SPA; dashboard dead letters now group by (task, exception class); /api/resources falls back to heartbeat snapshots cross-process; async-separation boundary now machine-checkable; run_maybe_async raises a clear taskito-specific error under a running loop; Redis status discriminants sourced from JobStatus enum (compile-fails on enum reorder); enqueue_batch Rust signature widened to Option<Vec<Option<i32>>>.

Fixed — scheduler concurrency cap atomicity (TOCTOU + off-by-one); scheduler reschedule on full/closed worker channel; enqueue_many middleware contract symmetry (per-job args, mutation propagation, exception logging); result() / aresult() no longer mask terminal failures as TimeoutError; result_handler Failure branch deduped from 3× get_job to 1×; ResourcePool._active_count underflow on factory failure; prefork timeout kill; CI PyO3 finalization SIGABRT; dashboard timestamp render (year 58282); dashboard DAG cycle bound; dashboard settings storage opacity; dashboard dead-letter row keyboard accessibility.

Internal — app.py, workflows/tracker.py, redis_backend/jobs.rs, py_queue/workflow_ops.rs, and dashboard.py all split into packages/submodules; dashboard health-audit follow-ups (extracted pure modules, vitest coverage on api-client / errors / settings / refresh-interval, useExhaustiveDependencies promoted to error); dependency bumps (redis 0.27 → 1.2, libsqlite3-sys 0.30 → 0.37, thiserror 1 → 2, rand 0.8 → 0.10, etc.).

Test counts at release

• Rust: 89 (was 78)
• Python: 496 passed, 9 skipped across 49 files (was 469 / 46)
• Dashboard (vitest): 81

Test plan

• cargo check --workspace clean
• cargo test --workspace — 89 passed
• cargo check --workspace --features postgres — clean
• cargo check --workspace --features redis — clean
• uv run python -m pytest tests/python/ -v — 496 passed
• uv run ruff check py_src/ tests/ — clean
• uv run mypy py_src/taskito/ --no-incremental — clean
• grep -rn "0\\.11\\.1" --include="*.toml" --include="*.py" --include="*.lock" returns empty (excluding the historical ## 0.11.1 heading in the changelog)

After merge

This PR only bumps the version. The release tag (v0.12.0) and PyPI / crates.io publish are deliberate follow-up steps, not part of this PR.