Implement GET /api/today endpoint (P1.4)

Add the core daily snapshot API that powers the dashboard. Returns:
- Training decision (status, reason, icon) using decision engine
- Cycle data (cycleDay, phase, phaseConfig, daysUntilNextPhase)
- Biometrics (hrvStatus, bodyBattery, weekIntensity, phaseLimit)
- Nutrition guidance (seeds, carbRange, ketoGuidance)

When no DailyLog exists (Garmin not synced), returns sensible defaults:
hrvStatus="Unknown", bodyBattery=100, weekIntensity=0. This allows
the app to function without Garmin integration.

22 tests covering auth, validation, all decision paths, override
handling, phase-specific logic, and nutrition guidance.

🤖 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

@@ -2,7 +2,106 @@
 // ABOUTME: Returns complete daily snapshot with decision, biometrics, and nutrition.
 import { NextResponse } from "next/server";
 
-export async function GET() {
-  // TODO: Implement today's data retrieval
-  return NextResponse.json({ message: "Not implemented" }, { status: 501 });
-}
+import { withAuth } from "@/lib/auth-middleware";
+import {
+  getCycleDay,
+  getPhase,
+  getPhaseConfig,
+  getPhaseLimit,
+  PHASE_CONFIGS,
+} from "@/lib/cycle";
+import { getDecisionWithOverrides } from "@/lib/decision-engine";
+import { getNutritionGuidance } from "@/lib/nutrition";
+import { createPocketBaseClient } from "@/lib/pocketbase";
+import type { DailyData, DailyLog } from "@/types";
+
+// Default biometrics when no Garmin data is available
+const DEFAULT_BIOMETRICS = {
+  hrvStatus: "Unknown" as const,
+  bodyBatteryCurrent: 100,
+  bodyBatteryYesterdayLow: 100,
+  weekIntensityMinutes: 0,
+};
+
+export const GET = withAuth(async (_request, user) => {
+  // Validate required user data
+  if (!user.lastPeriodDate) {
+    return NextResponse.json(
+      {
+        error:
+          "User has no lastPeriodDate set. Please log your period start date first.",
+      },
+      { status: 400 },
+    );
+  }
+
+  // Calculate cycle information
+  const cycleDay = getCycleDay(
+    new Date(user.lastPeriodDate),
+    user.cycleLength,
+    new Date(),
+  );
+  const phase = getPhase(cycleDay);
+  const phaseConfig = getPhaseConfig(phase);
+  const phaseLimit = getPhaseLimit(phase);
+
+  // Calculate days until next phase
+  const currentPhaseIndex = PHASE_CONFIGS.findIndex((c) => c.name === phase);
+  const nextPhaseIndex = (currentPhaseIndex + 1) % PHASE_CONFIGS.length;
+  const nextPhaseStartDay = PHASE_CONFIGS[nextPhaseIndex].days[0];
+
+  let daysUntilNextPhase: number;
+  if (nextPhaseIndex === 0) {
+    // Currently in LATE_LUTEAL, next phase is MENSTRUAL (start of new cycle)
+    daysUntilNextPhase = user.cycleLength - cycleDay + 1;
+  } else {
+    daysUntilNextPhase = nextPhaseStartDay - cycleDay;
+  }
+
+  // Try to fetch today's DailyLog for biometrics
+  let biometrics = { ...DEFAULT_BIOMETRICS, phaseLimit };
+  try {
+    const pb = createPocketBaseClient();
+    const today = new Date().toISOString().split("T")[0];
+    const dailyLog = await pb
+      .collection("dailyLogs")
+      .getFirstListItem<DailyLog>(`user="${user.id}" && date~"${today}"`);
+
+    biometrics = {
+      hrvStatus: dailyLog.hrvStatus,
+      bodyBatteryCurrent: dailyLog.bodyBatteryCurrent,
+      bodyBatteryYesterdayLow: dailyLog.bodyBatteryYesterdayLow,
+      weekIntensityMinutes: dailyLog.weekIntensityMinutes,
+      phaseLimit: dailyLog.phaseLimit,
+    };
+  } catch {
+    // No daily log found - use defaults
+  }
+
+  // Build DailyData for decision engine
+  const dailyData: DailyData = {
+    hrvStatus: biometrics.hrvStatus,
+    bbYesterdayLow: biometrics.bodyBatteryYesterdayLow,
+    phase,
+    weekIntensity: biometrics.weekIntensityMinutes,
+    phaseLimit: biometrics.phaseLimit,
+    bbCurrent: biometrics.bodyBatteryCurrent,
+  };
+
+  // Get training decision with override handling
+  const decision = getDecisionWithOverrides(dailyData, user.activeOverrides);
+
+  // Get nutrition guidance
+  const nutrition = getNutritionGuidance(cycleDay);
+
+  return NextResponse.json({
+    decision,
+    cycleDay,
+    phase,
+    phaseConfig,
+    daysUntilNextPhase,
+    cycleLength: user.cycleLength,
+    biometrics,
+    nutrition,
+  });
+});