feat!: scope chat model configs to organizations (#27955)

## Summary

Scope chat model configurations and runtime model resolution to
organizations. Each model configuration now belongs to exactly one
organization, each organization can have its own default model, and
Chatd only uses models that belong to the chat's organization.

Depends on #27968.

## Problem

Chat model configurations are currently deployment-wide even though
chats belong to organizations. A default model, explicitly selected
model, or configured model override can therefore reference the same
model configuration from every organization. This prevents model
catalogs and defaults from being isolated by organization and allows a
chat to retain or select a model that its organization does not own.

Existing model configurations also have no organization ownership or
organization-scoped ACL data, and the database enforces a single live
default across the entire deployment.

## Fix

Add required organization ownership and ACL storage to chat model
configurations. The migration assigns every existing configuration to
the default organization, seeds organization-wide read access, and
replaces the deployment-wide default constraint with a per-organization
constraint.

Scope Chatd lookup, admission, fallback, and queue-promotion behavior to
the chat's organization:

- New chats use the usable default model from their organization.
- Explicit model selections from another organization are rejected.
- Deployment or personal overrides that point to a foreign model are not
used for the chat.
- Historical cross-organization model references remain unchanged in the
database, but runtime generation falls back to a usable local default.
- Queue promotion resolves unavailable, disabled, missing, or foreign
stored models to the usable local default and fails when no usable local
default exists.

The existing model configuration management and availability routes
remain compatibility routes for the default organization in this layer.
A later layer in the stack adds organization-scoped management routes
and updates clients to use them.

## Breaking change

This changes chat model behavior from deployment-wide to
organization-scoped. Existing configurations are migrated only to the
default organization; they are not copied into every organization.

As a result:

- A non-default organization no longer inherits the deployment's
existing default model.
- Creating a chat in an organization without a usable local default
fails with `400: No default chat model config is configured.`
- Explicitly selecting a configuration owned by another organization
fails with `400: Model config does not belong to the specified
organization.`
- Existing chats whose stored model belongs to another organization can
generate only if a usable local default is available; otherwise
generation fails.

Deployments using Agents in non-default organizations must configure a
usable default chat model separately for each organization after the
organization-scoped management layers in this stack are available.

_This pull request description was generated by Coder Agents._

coderd/database/dump.sql

@@ -2077,9 +2077,14 @@ CREATE TABLE chat_model_configs (
     compression_threshold integer NOT NULL,
     options jsonb DEFAULT '{}'::jsonb NOT NULL,
     ai_provider_id uuid,
+    organization_id uuid NOT NULL,
+    group_acl jsonb DEFAULT '{}'::jsonb NOT NULL,
+    user_acl jsonb DEFAULT '{}'::jsonb NOT NULL,
     CONSTRAINT chat_model_configs_ai_provider_required_when_active CHECK (((deleted = true) OR (ai_provider_id IS NOT NULL))),
     CONSTRAINT chat_model_configs_compression_threshold_check CHECK (((compression_threshold >= 0) AND (compression_threshold <= 100))),
-    CONSTRAINT chat_model_configs_context_limit_check CHECK ((context_limit > 0))
+    CONSTRAINT chat_model_configs_context_limit_check CHECK ((context_limit > 0)),
+    CONSTRAINT chat_model_configs_group_acl_is_object CHECK ((jsonb_typeof(group_acl) = 'object'::text)),
+    CONSTRAINT chat_model_configs_user_acl_is_object CHECK ((jsonb_typeof(user_acl) = 'object'::text))
 );
 
 CREATE SEQUENCE chat_queued_messages_position_seq
@@ -4874,7 +4879,9 @@ CREATE INDEX idx_chat_model_configs_ai_provider_id ON chat_model_configs USING b
 
 CREATE INDEX idx_chat_model_configs_enabled ON chat_model_configs USING btree (enabled);
 
-CREATE UNIQUE INDEX idx_chat_model_configs_single_default ON chat_model_configs USING btree ((1)) WHERE ((is_default = true) AND (deleted = false));
+CREATE INDEX idx_chat_model_configs_organization_id ON chat_model_configs USING btree (organization_id);
+
+CREATE UNIQUE INDEX idx_chat_model_configs_single_default ON chat_model_configs USING btree (organization_id) WHERE ((is_default = true) AND (deleted = false));
 
 CREATE INDEX idx_chat_queued_messages_chat_id ON chat_queued_messages USING btree (chat_id);
 
@@ -5256,6 +5263,9 @@ ALTER TABLE ONLY chat_model_configs
 ALTER TABLE ONLY chat_model_configs
     ADD CONSTRAINT chat_model_configs_created_by_fkey FOREIGN KEY (created_by) REFERENCES users(id);
 
+ALTER TABLE ONLY chat_model_configs
+    ADD CONSTRAINT chat_model_configs_organization_id_fkey FOREIGN KEY (organization_id) REFERENCES organizations(id) ON DELETE CASCADE;
+
 ALTER TABLE ONLY chat_model_configs
     ADD CONSTRAINT chat_model_configs_updated_by_fkey FOREIGN KEY (updated_by) REFERENCES users(id);