Replace ateredis with atepg (#940)

## Summary

A follow up to #640 where we introduced PostgreSQL as an alternative
storage backend, selected conditionally in ateapi.

- Deleted ateredis, its tests, and its dependencies
- Removed Redis backend selection and configuration so ateapi always
connects to Postgres
- Replaced Valkey resources with Postgres in the standard and Kind
deployment paths and simplified install script
- Replaced miniredis fixtures with isolated Postgres testcontainers and
added centralized helpers for seeding resources
- Renamed Redis-specific debug flush command to backend-neutral
`debug-clear-store` in CLI
- Updated comments and docs where applicable

## Benchmarking

Extensive benchmarking have been performed to evaluate Redis vs
Postgres, and results can be found in these two documents:

-
https://docs.google.com/document/d/10K0wB6aTeFkJCL4HN3NbLJCdFGoLYdhIcqkFnqFHkKc/edit?usp=sharing
-
https://docs.google.com/document/d/12-ko_BFHcBo_nJkx9f4B7zMbiiWKC2saGhMhZG3aQ-s/edit?usp=sharing

---------

Signed-off-by: Jet Chiang <pokyuen.jetchiang-ext@solo.io>

.github/workflows/pr-workflow.yaml

@@ -78,7 +78,7 @@ jobs:
     - name: Create cluster
       run: hack/create-kind-cluster.sh
     - name: Install Agent Substrate
-      run: hack/install-ate-kind.sh --deploy-ate-system --store-backend=postgres
+      run: hack/install-ate-kind.sh --deploy-ate-system
     - name: Deploy micro-VM counter demo
       # Stages the (cached) assets into the cluster's rustfs and applies the
       # counter-microvm demo onto the control plane installed above.