[DASH-API-01] Implement dashboard analytics API

[Krisor1986]

Goal

Provide calculated data for dashboard KPIs and charts.

Tasks

• Define date-range parameters
• Calculate leads, conversions and active clients
• Calculate jobs and appointments
• Calculate quotations and revenue values in euros
• Return comparisons with the previous period
• Support day, month and year grouping

Acceptance criteria

• Dashboard views do not calculate business metrics locally
• Results respect the selected period
• All analytics are company isolated

Dependencies

• [DASH-01] Define dashboard metrics and interaction rules #12
• [LEAD-API-01]
• [CLIENT-API-01]
• [JOB-API-01]
• [AGENDA-API-01]