logging: clearer chain-provider startup summary and backend tags

[LandynDev]

What

Make validator/miner chain-provider logging legible: a one-time startup summary of which API each provider talks to, and consistent backend tags on the RPC/verify path so it's clear who served a call.

Why

Today the backend in use is scattered across ad-hoc inline prefixes (BTC RPC ..., BTC Esplora: ..., TAO scan: ...) and there's no single "here's the lineup" line at load. Hard to tell at a glance which API a node is using or which Esplora host served a given lookup.

Changes

• Startup summary (create_chain_providers, on check=True): logs each provider and its backend once, e.g.

  Chain providers ready:
    btc → Core RPC http://localhost:8332 (primary) + Esplora fallback: blockstream.info, mempool.space
    tao → Subtensor wss://entrypoint-finney…:443
  

  Driven by a new describe() on ChainProvider (default = chain name; overridden by BTC/TAO).
• Backend tags on the verify + RPC-call path: [BTC-RPC], [Esplora], [Subtensor] (module constants, defined once).
• Short host tags in Esplora failover narration — full URLs replaced with a compact registrable label so repeated lines don't pollute:

  Esplora [1/2] gomaestro-api/tx/9f3a…0f1a → rate-limited (429); falling back to: blockstream
  Esplora [2/2] blockstream/tx/9f3a…0f1a → 200 (served after 1 fallback(s))
  

Scope

Limited to the transaction-verification / RPC-call path — the "which API served this" story. Send/sign/address/fee log lines are intentionally untouched. No behavior changes; logging strings only. Full base URL is still preserved inside the raised HTTPError for deep debugging.

Test

ruff format + ruff check pass; package compiles. No tests assert on the changed log strings.