Implement POST/DELETE /api/overrides endpoints (P1.5)

Add override management API for the training decision system:
- POST /api/overrides adds an override (flare, stress, sleep, pms)
- DELETE /api/overrides removes an override
- Both endpoints use withAuth middleware
- Validation for override types, idempotent operations
- 14 tests covering auth, validation, and persistence

Also fix type error in today/route.ts where DailyLog body battery
fields could be null but biometrics object expected numbers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app/api/today/route.ts

@@ -13,11 +13,16 @@ import {
 import { getDecisionWithOverrides } from "@/lib/decision-engine";
 import { getNutritionGuidance } from "@/lib/nutrition";
 import { createPocketBaseClient } from "@/lib/pocketbase";
-import type { DailyData, DailyLog } from "@/types";
+import type { DailyData, DailyLog, HrvStatus } from "@/types";
 
 // Default biometrics when no Garmin data is available
-const DEFAULT_BIOMETRICS = {
-  hrvStatus: "Unknown" as const,
+const DEFAULT_BIOMETRICS: {
+  hrvStatus: HrvStatus;
+  bodyBatteryCurrent: number;
+  bodyBatteryYesterdayLow: number;
+  weekIntensityMinutes: number;
+} = {
+  hrvStatus: "Unknown",
   bodyBatteryCurrent: 100,
   bodyBatteryYesterdayLow: 100,
   weekIntensityMinutes: 0,
@@ -69,8 +74,11 @@ export const GET = withAuth(async (_request, user) => {
 
     biometrics = {
       hrvStatus: dailyLog.hrvStatus,
-      bodyBatteryCurrent: dailyLog.bodyBatteryCurrent,
-      bodyBatteryYesterdayLow: dailyLog.bodyBatteryYesterdayLow,
+      bodyBatteryCurrent:
+        dailyLog.bodyBatteryCurrent ?? DEFAULT_BIOMETRICS.bodyBatteryCurrent,
+      bodyBatteryYesterdayLow:
+        dailyLog.bodyBatteryYesterdayLow ??
+        DEFAULT_BIOMETRICS.bodyBatteryYesterdayLow,
       weekIntensityMinutes: dailyLog.weekIntensityMinutes,
       phaseLimit: dailyLog.phaseLimit,
     };