feat: migrate persistence from PostgreSQL/Ecto to VeriSimDB

Rewrite DbStore to use VeriSimDB octad API instead of Ecto/Repo.
Extend VeriSimDB.Client with generic CRUD operations (create_octad,
get_octad, update_octad, delete_octad, list_octads). Remove ecto_sql
and postgrex dependencies. Clean Ecto config from config.exs and
dev.exs. Disable Repo supervisor in application.ex. Public DbStore API
unchanged — callers (NativeBridge, Auth, SettingsStore) need no changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

backend/config/config.exs

@@ -7,16 +7,10 @@
 # General application configuration
 import Config
 
-config :stapeln,
-  ecto_repos: [Stapeln.Repo],
-  generators: [timestamp_type: :utc_datetime]
-
-config :stapeln, Stapeln.Repo,
-  database: "stapeln_dev",
-  username: "postgres",
-  password: "postgres",
-  hostname: "localhost",
-  port: 5432
+config :stapeln, []
+
+# VeriSimDB instance for stapeln data (port 8093)
+# Set VERISIMDB_URL=http://localhost:8093 in your environment
 
 config :stapeln, :api_auth,
   enabled: true,

backend/config/dev.exs

@@ -1,16 +1,5 @@
 import Config
 
-# Configure the Repo for development
-config :stapeln, Stapeln.Repo,
-  database: "stapeln_dev",
-  username: "postgres",
-  password: "postgres",
-  hostname: "localhost",
-  port: 5432,
-  stacktrace: true,
-  show_sensitive_data_on_connection_error: true,
-  pool_size: 10
-
 # For development, we disable any cache and enable
 # debugging and code reloading.
 #

backend/lib/stapeln/application.ex

@@ -7,23 +7,11 @@ defmodule Stapeln.Application do
 
   @impl true
   def start(_type, _args) do
-    # Optionally start the Ecto Repo when ecto_sql is available and
-    # PostgreSQL is configured. Falls back to GenServer stores otherwise.
-    repo_children =
-      if Code.ensure_loaded?(Stapeln.Repo) do
-        [Stapeln.Repo]
-      else
-        []
-      end
-
     children =
       [
         StapelnWeb.Telemetry,
         {DNSCluster, query: Application.get_env(:stapeln, :dns_cluster_query) || :ignore},
-        {Phoenix.PubSub, name: Stapeln.PubSub}
-      ] ++
-        repo_children ++
-        [
+        {Phoenix.PubSub, name: Stapeln.PubSub},
           Stapeln.StackStore,
           Stapeln.PipelineStore,
           Stapeln.Auth.UserStore,