feat(backend): add PoolProxy, Querier, and per-test transaction infrastructure

New types and helpers for context-aware DB routing and per-test transactions:

- PoolProxy: wraps pgxpool.Pool, routes queries through context transaction when active
- Querier: interface accepted by fixture/helper functions for decoupling
- ContextWithTx / TxFromContext: store/extract pgx.Tx in context.Context
- SetupTestTx (in testutils): begins tx, stores in context, auto-rolls back on cleanup
- SetupTestTx (in testtx): package-level variant with semaphore for parallel safety

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/db/context.go

@@ -0,0 +1,24 @@
+package db
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+)
+
+type ctxKey string
+
+const txCtxKey ctxKey = "poolproxy_tx"
+
+// ContextWithTx stores a pgx.Tx in the context for PoolProxy routing.
+// When PoolProxy.Exec/Query/QueryRow sees this context key, it routes the
+// call through the stored transaction instead of the pool.
+func ContextWithTx(ctx context.Context, tx pgx.Tx) context.Context {
+	return context.WithValue(ctx, txCtxKey, tx)
+}
+
+// TxFromContext extracts a pgx.Tx from context (returns nil if none active).
+func TxFromContext(ctx context.Context) pgx.Tx {
+	tx, _ := ctx.Value(txCtxKey).(pgx.Tx)
+	return tx
+}