Add monitoring service and database schema

- Implement monitoring service with heartbeat, lease tasks, resume tasks, and task result handling.
- Create monitoring time utilities for business date calculations.
- Add unit tests for date window resolution and business day handling.
- Define database schema for monitoring-related tables including quotas, daily reports, and task management.
- Establish migration scripts for creating and dropping monitoring tables.

server/migrations/20260410103000_create_monitoring_tables.down.sql

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+DROP TABLE IF EXISTS tenant_monitoring_quotas;

server/migrations/20260410103000_create_monitoring_tables.up.sql

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+CREATE TABLE tenant_monitoring_quotas (
+    tenant_id                BIGINT PRIMARY KEY REFERENCES tenants(id),
+    max_brands               INT NOT NULL DEFAULT 1,
+    collection_mode          VARCHAR(20) NOT NULL DEFAULT 'plugin',
+    question_selection_mode  VARCHAR(20) NOT NULL DEFAULT 'best_effort_all',
+    collect_frequency        VARCHAR(20) NOT NULL DEFAULT 'daily',
+    enabled_platforms        JSONB NOT NULL DEFAULT '["deepseek","qwen","doubao"]'::jsonb,
+    primary_installation_id  BIGINT REFERENCES plugin_installations(id),
+    task_daily_hard_cap      INT NOT NULL DEFAULT 36,
+    plan_tier                VARCHAR(20) NOT NULL DEFAULT 'free',
+    created_at               TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at               TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);