feat(O1): instrument ingest handler with Prometheus metrics

- Track ingest requests, errors, auth failures, rate limits, duplicates
- Track bytes ingested, records queued
- In-flight gauge for concurrent ingest jobs
- Timer for ingest duration histogram
- Metrics: I1-I12 (12 metrics instrumented)

.env

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+# Local development environment (.env file)
+# Copy to .env and fill in your local/dev URLs
+# .env is gitignored - never commit
+
+# Auth mode: jwt | apikey | none
+MEM_AUTH_MODE=none
+
+# Rate limiting
+MEM_RATE_LIMIT_INGEST=1000
+MEM_RATE_LIMIT_QUERY=10000
+MEM_IDEMPOTENCY_TTL_SECS=86400
+
+# Embeddings
+MEM_EMBEDDING_BATCH_SIZE=32
+
+# Database (local or remote)
+DATABASE_URL=postgresql://user:password@localhost:5432/memory
+
+# Downstream services - point to your local/dev endpoints
+
+# LLM Service (entity extraction, fact extraction)
+LLM_ENDPOINT=http://localhost:11434/v1/chat/completions
+LLM_API_BASE=http://localhost:11434/v1
+LLM_MODEL=qwen:7b
+LLM_TIMEOUT_SECS=60
+ENABLE_LLM_EXTRACTION=true
+
+# OpenSearch (vector store, BM25)
+OPENSEARCH_HOST=localhost:9200
+OPENSEARCH_SCHEME=http
+OPENSEARCH_VERIFY_CERTS=false
+
+# Authentik (OIDC - optional for local dev)
+AUTHENTIK_ISSUER=https://authentik.riotpiao.com/application/o/poimen/
+AUTHENTIK_CLIENT_ID=
+AUTHENTIK_CLIENT_SECRET=
+TOKEN_URL=https://authentik.riotpiao.com/application/o/token/
+AUTHENTIK_VERIFY_SSL=false
+
+# Temporal (workflow orchestration - future)
+TEMPORAL_ENDPOINT=localhost:7233
+TEMPORAL_NAMESPACE=poimen
+
+# API Gateway (route optimization - future)
+GATEWAY_URL=http://localhost:8080
+
+# Server config
+MEM_PORT=8080
+MEM_API_KEY=test-key
+MEM_HOME=/tmp