Improvements to Stirling Engine to prepare for SaaS release (#6603)

# Description of Changes
- Use pool for postgres connections
- Add ability to require user ID to be set on API calls to the engine
- Add process-wide concurrency cap on AI access (in addition to existing
user caps)
- Allow number of workers (threads) to be specified for stirling engine
- Update env var names to reflect that the DB is not just for RAG

engine/uv.lock

@@ -607,7 +607,7 @@ dependencies = [
     { name = "opentelemetry-sdk" },
     { name = "pgvector" },
     { name = "posthog" },
-    { name = "psycopg", extra = ["binary"] },
+    { name = "psycopg", extra = ["binary", "pool"] },
     { name = "pydantic" },
     { name = "pydantic-ai" },
     { name = "pydantic-ai-slim", extra = ["voyageai"] },
@@ -633,7 +633,7 @@ requires-dist = [
     { name = "opentelemetry-sdk", specifier = ">=1.39.0" },
     { name = "pgvector", specifier = ">=0.3.6" },
     { name = "posthog", specifier = ">=3.0.0" },
-    { name = "psycopg", extras = ["binary"], specifier = ">=3.2" },
+    { name = "psycopg", extras = ["binary", "pool"], specifier = ">=3.2" },
     { name = "pydantic", specifier = ">=2.0.0" },
     { name = "pydantic-ai", specifier = ">=1.67.0" },
     { name = "pydantic-ai-slim", extras = ["voyageai"], specifier = ">=1.67.0" },
@@ -2165,6 +2165,9 @@ wheels = [
 binary = [
     { name = "psycopg-binary", marker = "implementation_name != 'pypy'" },
 ]
+pool = [
+    { name = "psycopg-pool" },
+]
 
 [[package]]
 name = "psycopg-binary"
@@ -2195,6 +2198,18 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/98/5a/291d89f44d3820fffb7a04ebc8f3ef5dda4f542f44a5daea0c55a84abf45/psycopg_binary-3.3.3-cp314-cp314-win_amd64.whl", hash = "sha256:165f22ab5a9513a3d7425ffb7fcc7955ed8ccaeef6d37e369d6cc1dff1582383", size = 3652796, upload-time = "2026-02-18T16:52:14.02Z" },
 ]
 
+[[package]]
+name = "psycopg-pool"
+version = "3.3.1"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "typing-extensions" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/90/82/7a23d26039827ecd4ebe93905651029ddd307c5182ad59296dfb6f67b528/psycopg_pool-3.3.1.tar.gz", hash = "sha256:b10b10b7a175d5cc1592147dc5b7eec8a9e0834eb3ed2c4a92c858e2f51eb63c", size = 31661, upload-time = "2026-05-01T23:31:59.809Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/37/ed/89c2c620af0e1660354cd8aabf9f5b21f911597ce22acb37c805d6c86bc8/psycopg_pool-3.3.1-py3-none-any.whl", hash = "sha256:2af5b432941c4c9ad5c87b3fa410aec910ec8f7c122855897983a06c45f2e4b5", size = 40023, upload-time = "2026-05-01T23:31:53.136Z" },
+]
+
 [[package]]
 name = "py-key-value-aio"
 version = "0.4.4"