feat: structured notes + LogLevel enum

[pratyush618]

Summary

Adds two related additions to the public surface:

• LogLevel enum for structured task logs. JobContext.log() and Queue.query_logs() now take a LogLevel instead of a free-form string. JobContext.publish() writes with LogLevel.RESULT. Subclasses str so the wire format to the Rust column is unchanged. Exported from the package root.
• Structured notes field on jobs. New notes: dict | None kwarg on Queue.enqueue(), Queue.enqueue_many() (uniform + per-job notes_list), and TaskWrapper.apply_async(). Validated at the Python boundary against a tight contract: ≤ 15 top-level fields, ≤ 64-char keys, ≤ 500-char string leaves, ≤ 3 levels nesting, ≤ 4 KiB encoded. Stored as a canonical JSON-encoded string in a new notes column on jobs, dead_letter, and archived_jobs for SQLite + Postgres, and on the Redis Job/DeadJobEntry JSON. Read back as a parsed dict via JobResult.notes. Survives DLQ round-trips and replay. Distinct from the existing free-form metadata string blob.

Why notes ≠ metadata

metadata is a power-user escape hatch — opaque string, unbounded, unstructured. notes is the dashboard-renderable equivalent for user-readable annotations (customer IDs, business reasons, priority comments). Bounded field count means the dashboard can render the full set as a fixed-size key/value table.

Implementation outline

Layer	What changed
Rust core	New notes: Option<String> on Job / NewJob / JobRow / NewJobRow / DeadLetterRow / ArchivedJobRow / DeadJob. New notes TEXT column added to create_tables() and alter_statements() (idempotent on Postgres via IF NOT EXISTS). All call-sites updated (diesel_common/jobs.rs, sqlite/postgres dead_letter + archival, redis backend, scheduler, workflow ops).
PyO3 bindings	notes parameter on enqueue/enqueue_batch signatures; notes getter on PyJob; type stubs updated.
Python	New taskito.notes module with validate_and_encode_notes + size/depth/key constants. New NotesValidationError (subclasses both TaskitoError and ValueError). Wired through Queue.enqueue/enqueue_many, TaskWrapper.apply_async, JobResult.notes/to_dict.
Dashboard	notes on the Job DTO; new NotesCard component on the job overview tab rendering as a key/value table (falls back to raw <pre> if the column can't be parsed).
Docs	New docs/content/docs/guides/observability/notes.mdx guide; notes documented on queue.enqueue / enqueue_many reference. LogLevel docs refreshed in the existing logging guide and streaming/context references.

Cross-backend parity verified by the existing contract suite at crates/taskito-core/tests/rust/storage_tests.rs (Postgres + Redis kick in via CI when TASKITO_POSTGRES_TEST_URL / TASKITO_REDIS_TEST_URL are set).

Test plan

• cargo check --workspace (default / +postgres / +redis / +native-async) — clean
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• cargo test --workspace — 91 tests pass, includes 2 new SQLite tests covering notes round-trip and DLQ retry preservation
• uv run maturin develop — wheel builds clean
• uv run python -m pytest tests/ — 680 passed, 9 skipped (21 new tests in tests/core/test_notes.py)
• uv run ruff check py_src/ tests/ — clean
• uv run ruff format --check py_src/ tests/ — clean
• uv run mypy py_src/taskito/ tests/ --no-incremental — clean (203 files)
• pnpm --dir dashboard lint / typecheck / test — 97 vitest tests pass, biome + tsc clean
• pnpm --dir docs lint — clean