feat(vibe): weigh local calendar context in session planning

The director had no idea what hour or season a session started in, so a
22:00 weeknight and a Sunday morning drew from the same pool. The client
now sends localHour, weekday, month and an optional timeZone; the route
validates and bounds all four, the coordinator threads them through, and
the generators use them as scoring signals.

Also exposes GET /library/stats, which the Home page counts read.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/src/services/vibe-session-coordinator.service.ts

@@ -1,6 +1,6 @@
 import { DbService, VibeEvent, VibePlan, VibeSession } from './db.service.js';
 import { SessionDirector } from './session-director.service.js';
-import { Candidate } from './generators.service.js';
+import { Candidate, VibeCalendarContext } from './generators.service.js';
 
 /**
  * This is deliberately a narrow bridge between the durable Vibe ledger and
@@ -22,6 +22,7 @@ export type VibeEventType = (typeof VIBE_EVENT_TYPES)[number];
 export interface StartVibeSessionInput {
   seedTrackId?: string;
   resumeSessionId?: string;
+  context?: VibeCalendarContext;
 }
 
 export interface AppendVibeEventInput {
@@ -70,10 +71,15 @@ export class VibeSessionCoordinator {
 
   async start(userId: string, input: StartVibeSessionInput): Promise<VibeSessionResponse> {
     if (input.resumeSessionId) return this.resume(userId, input.resumeSessionId);
-    // Vibe has no reliable device/activity/location signal. Start from neutral
-    // recommendation state and let actual listening behaviour shape the plan.
+    // Calendar context is a weak boot prior, never a substitute for listening
+    // feedback or long-term preference. It is intentionally coarse and is
+    // persisted with the session so a resume remains coherent.
+    const calendar = input.context;
+    const contextualEnergy = calendar && calendar.localHour < 6 ? 0.32
+      : calendar && calendar.localHour >= 18 && calendar.localHour < 23 ? 0.58
+        : 0.5;
     const initialState = {
-      energy: 0.5,
+      energy: contextualEnergy,
       noveltyHunger: 0.3,
       explorationCoefficient: 0.3,
       discoveryRadius: 0.38,
@@ -83,6 +89,7 @@ export class VibeSessionCoordinator {
       userId,
       policyVersion: DEFAULT_VIBE_POLICY_VERSION,
       seedTrackId: input.seedTrackId ?? null,
+      context: calendar ? { ...calendar } : undefined,
       profile: {
         goals: initialState.sessionGoal,
         explorationCoefficient: initialState.explorationCoefficient,
@@ -95,7 +102,7 @@ export class VibeSessionCoordinator {
     // session while the durable tables remain the source of truth.
     await this.db.createSessionState(
       userId,
-      undefined,
+      calendar ? JSON.stringify(calendar) : undefined,
       {
         energy: initialState.energy,
         noveltyHunger: initialState.noveltyHunger,
@@ -109,7 +116,7 @@ export class VibeSessionCoordinator {
       sessionId: session.id,
       userId,
       type: 'session_started',
-      payload: { policyVersion: DEFAULT_VIBE_POLICY_VERSION },
+      payload: { policyVersion: DEFAULT_VIBE_POLICY_VERSION, ...(calendar ? { calendar } : {}) },
     });
 
     const candidates = await this.director.buildPlan(userId, session.id, input.seedTrackId);