[Task]: NULL cost survives as null (f64 → Option<f64>)

Parent: #586 (Per-grain query APIs)

### Summary

Fix the NULL money bug: `UsageSeriesPoint.total_cost` must survive serialization as `null`, never `0.0` (governance#188).

### The bug

`crates/lightbridge-authz-usage/src/repo.rs:269`:
```rust
total_cost: row.total_cost.unwrap_or(0.0),
```
This collapses "unknown cost" into "$0.00". "I don't know what it cost" is not the same as "it was free."

### What to change

1. **`UsageSeriesPoint.total_cost`**: `f64` → `Option` in `crates/lightbridge-authz-usage/src/models/mod.rs:291`
2. **`repo.rs:269`**: remove the `unwrap_or(0.0)` — keep the `Option`
3. **Every new grain point struct** (`ExecutionSeriesPoint`, `DayFactSeriesPoint`): cost fields declared as `Option` (or `Option` for micro-USD BIGINT columns)

### Wire impact

`total_cost` changes from `number` to `number | null` in the JSON response. Existing callers that deserialize as `f64` will break on `null`. This is a deliberate, tested breaking change per governance#188.

### Cross-repo coordination

`converse-frontends` must be notified — the `total_cost` field changes from `number` to `number | null`. Feature-flag the frontend change if needed.

### Tests

- Existing `query_usage` test: seed a row with NULL cost → assert response has `null`, not `0.0`
- New grain tests (Tickets B/C/D): same pattern per grain
- `spend_query_it_tests` must pass unchanged (they already use `Option` for spend)

### Source of truth

- governance#188 — NULL cost must survive to the wire
- `docs/plans/0586-per-grain-query-apis-analysis.md` — AC 5

### Verification evidence

To be attached on the implementing PR.

Start with crates/lightbridge-authz-usage/src/models/mod.rs:291 and repo.rs:269, then read the existing query_usage test and spend_query_it_tests. Check the per-grain point structs and the tests described for Tickets B/C/D. Done means NULL costs serialize as null for each grain, the tests pass, and converse-frontends is notified of the breaking wire change.

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