feat(02-01): create analytics migration and analyticsService

- Migration 013: document_processing_events table with CHECK constraint on event_type
- Indexes on created_at (retention queries) and document_id (per-doc history)
- RLS enabled following migration 012 pattern
- analyticsService.recordProcessingEvent(): void return (fire-and-forget, never throws)
- analyticsService.deleteProcessingEventsOlderThan(): Promise<number> (retention delete)
- getSupabaseServiceClient() called per-method, never cached at module level

backend/src/models/migrations/013_create_processing_events_table.sql

@@ -0,0 +1,33 @@
+-- Migration: Create document_processing_events table for analytics
+-- Created: 2026-02-24
+-- Purpose: Establish analytics foundation for tracking document processing lifecycle events.
+--          Phase 2 of the monitoring feature — fire-and-forget instrumentation writes to
+--          this table without blocking the processing pipeline.
+
+-- =============================================================================
+-- TABLE: document_processing_events
+-- Records each lifecycle event during document processing for analytics and audit.
+-- Writes are always fire-and-forget (never awaited on the critical path).
+-- =============================================================================
+
+CREATE TABLE IF NOT EXISTS document_processing_events (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    document_id UUID NOT NULL,
+    user_id UUID NOT NULL,
+    event_type TEXT NOT NULL CHECK (event_type IN ('upload_started', 'processing_started', 'completed', 'failed')),
+    duration_ms INTEGER,                                         -- nullable: not applicable for all event types
+    error_message TEXT,                                          -- nullable: failure details for 'failed' events
+    stage TEXT,                                                  -- nullable: which processing stage the event occurred in
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Index required for 30-day retention queries (deleteProcessingEventsOlderThan)
+CREATE INDEX IF NOT EXISTS idx_document_processing_events_created_at
+    ON document_processing_events(created_at);
+
+-- Index for per-document event history queries
+CREATE INDEX IF NOT EXISTS idx_document_processing_events_document_id
+    ON document_processing_events(document_id);
+
+-- Enable RLS (service role key bypasses RLS automatically — explicit policies added in Phase 3)
+ALTER TABLE document_processing_events ENABLE ROW LEVEL SECURITY;

backend/src/services/analyticsService.ts

@@ -0,0 +1,88 @@
+import { getSupabaseServiceClient } from '../config/supabase';
+import { logger } from '../utils/logger';
+
+// =============================================================================
+// Types
+// =============================================================================
+
+export interface ProcessingEventData {
+  document_id: string;
+  user_id: string;
+  event_type: 'upload_started' | 'processing_started' | 'completed' | 'failed';
+  duration_ms?: number;
+  error_message?: string;
+  stage?: string;
+}
+
+// =============================================================================
+// recordProcessingEvent
+// =============================================================================
+
+/**
+ * Fire-and-forget analytics write for document processing lifecycle events.
+ *
+ * Return type is void (NOT Promise<void>) to prevent accidental await on the
+ * critical processing path. Any Supabase write failure is logged but never
+ * thrown — analytics must never block or break document processing.
+ *
+ * Architecture decision: Analytics writes are always fire-and-forget.
+ * See STATE.md: "Analytics writes are always fire-and-forget (never await on critical path)"
+ */
+export function recordProcessingEvent(data: ProcessingEventData): void {
+  const supabase = getSupabaseServiceClient();
+
+  void supabase
+    .from('document_processing_events')
+    .insert({
+      document_id: data.document_id,
+      user_id: data.user_id,
+      event_type: data.event_type,
+      duration_ms: data.duration_ms ?? null,
+      error_message: data.error_message ?? null,
+      stage: data.stage ?? null,
+      created_at: new Date().toISOString(),
+    })
+    .then(({ error }) => {
+      if (error) {
+        logger.error('analyticsService: failed to insert processing event', {
+          error: error.message,
+          document_id: data.document_id,
+          event_type: data.event_type,
+        });
+      }
+    });
+}
+
+// =============================================================================
+// deleteProcessingEventsOlderThan
+// =============================================================================
+
+/**
+ * Delete document_processing_events rows older than `days` days.
+ *
+ * Used by the retention cleanup job to enforce data retention policy.
+ * Returns the count of rows deleted.
+ *
+ * Follows the same pattern as HealthCheckModel.deleteOlderThan().
+ */
+export async function deleteProcessingEventsOlderThan(days: number): Promise<number> {
+  const cutoff = new Date(Date.now() - days * 86400000).toISOString();
+  const supabase = getSupabaseServiceClient();
+
+  const { data, error } = await supabase
+    .from('document_processing_events')
+    .delete()
+    .lt('created_at', cutoff)
+    .select();
+
+  if (error) {
+    logger.error('analyticsService: failed to delete old processing events', {
+      error: error.message,
+      days,
+      cutoff,
+    });
+    throw new Error(`failed to delete processing events older than ${days} days — ${error.message}`);
+  }
+
+  return data ? data.length : 0;
+}