chore(release): 0.13.0

[pratyush618]

Summary

Cut 0.13.0. Bumps versions across the workspace (`pyproject.toml`, all four crate `Cargo.toml`s, `py_src/taskito/init.py` fallback) and finalises the changelog by renaming the `## Unreleased` section to `## 0.13.0`. No code changes — exact same shape as the 0.12.3 release commit (#177).

Why a minor (not patch)

Three new public APIs and meaningful new capability ship in this release. Calling it a patch would mislead users into thinking there's no new surface to learn:

Added

• Workflow PostgreSQL backend (audit P1 closed)
• Workflow Redis backend
• Multi-backend workflow contract suite + CI parity
• `@queue.task(batch=…)` — producer-side task batching
• Saga compensation: `@queue.task(compensates=…)`, `Workflow.step(compensates=…)`, `current_compensation_context()`, 3 new `WorkflowState` + 3 new `WorkflowNodeStatus` + 6 new event types, schema additions for compensation tracking
• Compensator contract fully wired for static nodes (forward args / kwargs / result)
• Sub-workflow saga propagation with `MAX_SAGA_DEPTH=10` guard
• `PyJob.payload_bytes` getter
• Saga + batching guides, API reference, runnable examples

Fixed

• `expand_fan_out` atomicity (transactional batch insert)
• Diesel `?` → `$N` placeholder rewrite for Postgres workflow queries

Test plan

• `cargo test --workspace --features workflows` — all suites green
• `pnpm --dir docs build` — docs build clean (changelog 0.13.0 renders)
• Pre-commit gates green (ruff + mypy + clippy + fmt)

Tagging

Per the workflow, the tag will be created from the GitHub web UI after merge — this PR does not push a tag locally.