feat: v0.2 — batched quotes (get_quotes/1) and FX (get_fx_rate/2)

[fleveque]

Summary

Adds two functions on top of v0.1's single-symbol get_quote/1, both reusing the existing Session + HTTP wrapper.

YahooFinanceEx.get_quotes/1

{:ok, by_symbol} = YahooFinanceEx.get_quotes(["AAPL", "MSFT", "FAKE"])
by_symbol["AAPL"]      #=> {:ok, %YahooFinanceEx.Quote{...}}
by_symbol["FAKE"]      #=> {:error, :not_found}

• Internally chunks into batches of 50 (Yahoo's per-request ceiling) and merges results across batches.
• Dedupes the input list.
• Top-level errors (auth / transport / http_status) abort the whole call and bubble up; partial misses surface inside the result map as {:error, :not_found} for those keys — the top-level call still returns {:ok, map}.

YahooFinanceEx.get_fx_rate/2

{:ok, 1.08} = YahooFinanceEx.get_fx_rate("EUR", "USD")
{:ok, 1.0}  = YahooFinanceEx.get_fx_rate("USD", "USD")  # identity short-circuits, no HTTP

Uses Yahoo's <FROM><TO>=X ticker convention internally. Returns {:ok, rate} as a float().

Bookkeeping

• Version 0.1.0 → 0.2.0.
• CHANGELOG.md updated.
• README.md updated to v0.2 surface, dep snippet bumped to ~> 0.2.
• Package description tightened.

After merge

Manual step: mix hex.publish to push 0.2.0. The quantic dep bump + live price column lands in a follow-up PR on the quantic repo.

Test plan

• mix test clean (10 passing — 3 v0.1 + 7 new)
• mix compile --warnings-as-errors clean
• CI green