Adopt Tower & add function-style crawler API (extract / execute / store)

[oiwn]

Introduce a small Tower-based core so a crawler can run as a simple function while remaining fully composable (timeouts, retries, rate-limits, tracing). Provide crawl_once(extract, exec, store) that wires three Services together; supply adapters so users can pass plain async functions/closures.

Motivation:

• Keep ergonomics: “computation” as a function.
• Gain reliability + reuse: Tower layers for backoff, concurrency, timeouts, tracing.
• Easier testing: mock any stage with service_fn.
• Backend neutrality: queue/http/store are swappable.

Tasks

• Create capp-tower feature / crate.
• Implement crawl_once and core error CErr.
• Add adapters: closures → Service (service_fn).
• Provide example: in-memory extract, reqwest exec stack, Mongo store.
• Add tower-http tracing layer integration with tracing.
• Integration tests: happy path, timeout, retry, DLQ handoff.
• Bench: baseline vs layered exec (show overhead).
• Docs: quickstart + migration notes.