Implement LLM-WP-0008: provider-scoped account balance CLI

Add pluggable balance registry, OpenRouter credits/key limit client,
and llm-connect balance with one-shot --provider that does not change
library defaults.

contracts/functional/account-balance.md

@@ -0,0 +1,44 @@
+# Provider Account Balance
+
+`llm_connect.balance` reports **prepaid / key remaining** for a selected
+inference **backend**. Balance is never assumed to be OpenRouter unless that
+backend is selected (explicitly or as the current default provider).
+
+## Selection rules
+
+```python
+from llm_connect import get_account_balance, resolve_balance_provider
+
+resolve_balance_provider(None)           # → LLMConfig().provider (today openrouter)
+resolve_balance_provider("openrouter")   # one-shot; does not mutate defaults
+get_account_balance()                    # default backend
+get_account_balance("openrouter")        # explicit backend for this call only
+```
+
+- Explicit `provider` / CLI `--provider` is **invocation-only**.
+- It must not rewrite config files, env, `LLMConfig` class defaults, or
+  OpenRouter model defaults.
+- Unspecified → current default provider via `resolve_balance_provider(None)`.
+
+## Contract
+
+- `AccountBalance` carries optional wallet fields (`credits_*`) and optional
+  key-limit fields (`limit`, `limit_remaining`, `limit_reset`, `usage`).
+- EUR display fields use the same FX resolution as cost estimates.
+- `BalanceClientRegistry` maps provider id → client factory.
+- Unsupported providers raise `LLMBalanceUnsupportedError` listing supported
+  backends (do not fall back to OpenRouter).
+
+## OpenRouter (v1)
+
+- `GET /api/v1/credits` → account prepaid: remaining = total_credits − total_usage
+- `GET /api/v1/auth/key` → key limit remaining / reset policy
+- Both are fetched best-effort; one may fail while the other succeeds.
+
+## CLI
+
+```bash
+llm-connect balance
+llm-connect balance --provider openrouter
+llm-connect balance --json
+```