add nix flake devshell and process-compose

mirrors youngstartup setup. flake provides php 8.4, composer,
nodejs, postgresql, cypress, typescript, process-compose. shellHook
seeds a per-repo postgres cluster at .postgres/. process-compose
orchestrates postgres + backend (laravel) + vite (vue spa) for
local dev. .envrc auto-loads the flake via direnv. mailpit and
gitlab-ci-local omitted - not needed for tide blogging app.

process-compose.yaml

@@ -0,0 +1,37 @@
+version: "0.5"
+
+processes:
+  postgres:
+    command: postgres -D "$PGDATA" -k "$PGDATA" -c listen_addresses=127.0.0.1
+    shutdown:
+      signal: 2
+    readiness_probe:
+      exec:
+        command: pg_isready -h "$PGDATA"
+      initial_delay_seconds: 1
+      period_seconds: 2
+
+  backend:
+    command: php artisan serve --host=127.0.0.1 --port=8000
+    working_dir: ./backend
+    depends_on:
+      postgres:
+        condition: process_healthy
+    readiness_probe:
+      http_get:
+        host: 127.0.0.1
+        port: 8000
+        path: /up
+      initial_delay_seconds: 2
+      period_seconds: 10
+
+  vite:
+    command: npm run dev
+    working_dir: ./frontend/blog_portal
+    readiness_probe:
+      http_get:
+        host: 127.0.0.1
+        port: 5173
+        path: /
+      initial_delay_seconds: 2
+      period_seconds: 2