docs: refresh README — bump install version, document tracing feature, show Transaction::run example

[RAprogramm]

Problem

`README.md` has drifted from the current state of the crates:

1. Installation snippet pins `entity-derive = "0.4"`. Latest is `0.8`. New users copy-pasting will start on a 4-major-version-old API.
2. No mention of the `tracing` feature added in 0.8.0 (feat: opt-in tracing instrumentation on generated entity methods #118 / feat: opt-in tracing instrumentation on generated entity methods #119). Users have to dig into source to discover `features = ["tracing"]`.
3. No transaction example showing the current `Transaction::new(&pool).run(async |ctx| { ... })` signature. The Features table says "Transactions: Multi-entity atomic operations" but doesn't show what calling them looks like — and the API just changed in 0.7.0 (fix: Transaction::run() never commits — closure signature change #103).
4. No documented feature matrix for what each Cargo feature toggles (`postgres`, `clickhouse`, `mongodb`, `streams`, `api`, `validate`, `tracing`).

Fix

In `README.md`:

• Bump the Installation block to use `version = "0.8"`.
• Add a "Feature flags" section listing every Cargo feature with one-line descriptions.
• Add a "Transactions" subsection under Quick Reference with a copy-pasteable `.run(async |ctx| { ... })` example that uses the current API.
• Add a "Tracing" subsection showing how to enable the feature, what fields the spans carry, and a sample log line.

Out of scope

• Wiki updates (separate repo, separate cadence).
• Examples directory READMEs.

Acceptance

A new user reading the README top-to-bottom can:

• copy the Installation snippet and get the latest crate;
• find the list of optional features without grepping the codebase;
• see how to use transactions correctly (with the fixed `run` signature);
• enable structured logging in one feature flag.