Add architecture decision records for Discord.js, PostgreSQL, polling strategy, and modular monolith

docs/09_architecture_decisions/ADR-002.md

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+# ADR-002: PostgreSQL over MongoDB
+
+**Status:** Accepted  
+**Context:** Mileage data is relational (user → role → event → score). MongoDB adds flexibility but sacrifices query power.  
+**Decision:** Use PostgreSQL with JSONB columns for extensible user metadata.  
+**Consequences:** Schema migrations needed for new features; joins straightforward.