fix(discovery): read the session stats and duration the API actually sends

[jhosepmyr]

Problem

In production, the session history (Captura › Historial) did not show the Historias / Aceptadas / Pendientes / Preguntas columns. Every live session showed "—" as its duration. The session separator in Captura never showed "N historias".

Root cause

• Contract drift: DiscoverySessionResponse in the web expected storiesGeneratedCount, storiesAcceptedCount, pendingSuggestionsCount and questionsCount. The API sends storiesGenerated, storiesAccepted, suggestionsPending and questionsAsked, so showStats() was always false.
• Duration: the cell only read audioDurationMs. Only uploaded recordings set that field; live sessions never do.

Fix

• Model: uses the API field names and adds durationSeconds.
• History: reads the new names. The duration cell prefers durationSeconds and falls back to audioDurationMs for older deployments.
• Helpers: the logic moves to the pure helpers sessionDuration and hasSessionStats (history.helpers.ts).
• Captura: the session separator reads storiesGenerated.

Companion API PR: Kntro-Soft/reqsai-api#99. It makes durationSeconds report the audio length of uploads and makes storiesAccepted count accepted suggestions. Without it, this PR still shows the stats columns and live durations.

Tests

• ng test --watch=false: 40 files, 399 tests passed. That includes the new history.helpers.spec.ts (6 tests), also run alone.
• ng lint: clean.
• ng build: OK.