COD-52 fix: wire maxTodos + todoExpirationMinutes through ralph-config to the tracker

The Ralph settings modal sent maxTodos/todoExpirationMinutes but RalphConfigSchema
(zod) stripped them and the ralph-config route never applied them, so the inputs
were silent no-ops.

Fix: add both as optional positive-int fields to RalphConfigSchema; destructure
and apply them in the ralph-config route (matching the maxIterations pattern).
RalphTracker had no setters (the values were module constants) — added per-instance
_maxTodos/_todoExpiryMs (defaulting to the same constants, behavior unchanged),
switched the eviction + expiry sites to read them, and added
setMaxTodos/setTodoExpirationMinutes (minutes→ms) + getters.

Test: route test POSTs the two fields and asserts the route applies them to the
tracker. Verified RED (setters not called — fields stripped) → GREEN. 34/34
ralph-routes tests pass; tsc + eslint(src) + prettier + build clean. Frontend
already sent the fields (no change).

src/web/schemas.ts

@@ -497,6 +497,8 @@ export const RalphConfigSchema = z.object({
   enabled: z.boolean().optional(),
   completionPhrase: z.string().max(500).optional(),
   maxIterations: z.number().int().min(0).max(10000).optional(),
+  maxTodos: z.number().int().positive().max(10000).optional(),
+  todoExpirationMinutes: z.number().int().positive().max(525600).optional(),
   reset: z.union([z.boolean(), z.literal('full')]).optional(),
   disableAutoEnable: z.boolean().optional(),
 });