docs(adr): ADR-0003 choose Pub/Sub+Dataflow+BigQuery

Author: sahilgundu

adr/0003-use-pubsub-dataflow-bq.md

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+# ADR 0001: Choose Pub/Sub + Dataflow + BigQuery
+
+## Context
+We need low-latency FX streaming (quotes, trades, confirms) with managed ops, security (VPC-SC, CMEK), and tight GCP integration.
+
+## Options
+1) Kafka on GKE + Flink + self-managed warehouse
+2) Pub/Sub + Dataflow (Beam) + BigQuery (managed/serverless)
+
+## Decision
+Choose Pub/Sub + Dataflow + BigQuery.
+
+## Consequences
++ Lower ops toil, native CMEK/VPC-SC, quick scale.
+- More vendor lock-in vs Kafka/Flink portability.