fix(deploy): prevent App Engine request starvation under burst load

Prod cycled between 0-1 instances and returned site-wide 500/503 with
"Request was aborted after waiting too long to attempt to service your
request." Requests died in the pending queue (blank instanceId, ~2ms
latency), not in app code — App Engine never scaled out past one
instance despite max_instances=10.

Root cause: the scheduler had no visibility into real per-instance
concurrency (gunicorn -w 4), so it kept routing bursts to a single
saturated instance instead of spinning up more. min_instances=0 added
cold-start pile-ups on top.

- app.template.yaml: add max_concurrent_requests: 6 so the scheduler
  scales out before an instance saturates (2-worker headroom for bursts)
- CD_production.yml: MIN_INSTANCES 0 -> 1 to kill cold-start pile-ups
- CD_production.yml: gunicorn -w 4 -> -w 8 for more concurrency per F4

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jirhiker

.github/app.template.yaml

@@ -8,6 +8,7 @@ inbound_services:
 automatic_scaling:
   min_instances: ${MIN_INSTANCES}
   max_instances: ${MAX_INSTANCES}
+  max_concurrent_requests: 6
 handlers:
   - url: /.*
     secure: always

.github/workflows/CD_production.yml

@@ -148,8 +148,8 @@ jobs:
         run: |
           export MAX_INSTANCES="10"
           export SERVICE_NAME="ocotillo-api"
-          export ENTRYPOINT="gunicorn -w 4 -k uvicorn.workers.UvicornWorker main:app"
-          export MIN_INSTANCES="0"
+          export ENTRYPOINT="gunicorn -w 8 -k uvicorn.workers.UvicornWorker main:app"
+          export MIN_INSTANCES="1"
           envsubst < .github/app.template.yaml > app.yaml
 
       - name: Deploy to Google Cloud