Implement LLM-WP-0007: Kimi K3 default and EUR spend reporting

Add moonshotai/kimi-k3 as OpenRouter basemodel default after live smoke,
USD→EUR cost conversion, append-only usage ledger, and CLI run/cost/spend
week commands with token and euro reporting.

contracts/functional/usage-ledger.md

@@ -0,0 +1,33 @@
+# Usage Ledger and Spend Windows
+
+`llm_connect.usage` stores append-only spend events for operator cost reporting.
+It is separate from `QualityLedger` (adaptive routing quality signals).
+
+## UsageEvent
+
+Each event records provider, model, token counts, estimated `cost_usd` /
+`cost_eur`, cost/FX sources, call `source` (`cli` | `server` | `library`),
+and `recorded_at` (UTC).
+
+## Paths
+
+1. Explicit `--ledger` / constructor path
+2. Env `LLM_CONNECT_USAGE_LEDGER`
+3. `$XDG_DATA_HOME/llm-connect/usage.jsonl`
+4. `~/.local/share/llm-connect/usage.jsonl`
+
+Library and server auto-recording only runs when `LLM_CONNECT_USAGE_LEDGER` is
+set (opt-in). CLI `run` records by default to the resolved path unless
+`--no-ledger`.
+
+## Week windows
+
+`week_window("current"|"last", tz=...)` returns Monday-based half-open ranges
+in local time (default `Europe/Berlin`, override with `LLM_CONNECT_TZ` or
+`--tz`):
+
+- **current**: Monday 00:00 → now
+- **last**: previous Monday 00:00 → this Monday 00:00 (full Mon–Sun week)
+
+`UsageLedger.sum_range(start, end)` aggregates tokens and costs for
+`[start, end)`.