feat!: scope chat model override settings to organizations (#28442)

Moves chat model override settings from deployment scope to organization
scope, following the org-scoping of chat model configs in #28440.

Depends on #28440 (based on
`ethan/chat-models-availability-consolidation`).

Remote dogfood UAT passed on this change, including multi-organization
scenarios on a licensed deployment with real models.

## Problem

Chat model configs are now owned by organizations, but the override
settings that reference them stayed deployment-wide: admin
subagent/title/compaction overrides in `site_configs`, personal
overrides in `user_configs`, and the advisor model embedded in the
advisor runtime JSON. A single deployment-level override can only point
at one organization's model config, so overrides effectively worked for
the default organization only, and title generation had to soften its
hard-failure contract to tolerate unusable overrides.

## Fix

Admin overrides (general, explore, title generation, compaction,
advisor) and personal overrides (root, general, explore) become
organization-scoped rows in two new typed tables,
`chat_organization_model_overrides` and `chat_user_model_overrides`.
Composite foreign keys on `(organization_id, model_config_id)` make
cross-organization model references unrepresentable. The migration
deletes the legacy serialized `site_configs`/`user_configs` override
keys instead of migrating them; admins and users re-select models in the
new organization-scoped settings.

New endpoints live under
`/api/experimental/organizations/{organization}/chats/model-overrides`
(admin defaults) and
`/api/experimental/organizations/{organization}/members/{user}/chats/model-overrides`
(personal overrides); the legacy deployment-level routes are removed.
chatd resolves overrides via the chat's organization, restoring the
hard-failure contract for configured-but-unusable title generation
overrides. The deployment-level `AllowUsers` switch and advisor runtime
limits stay deployment-scoped.

The Coder Agents settings UI gains a per-organization "Defaults &
overrides" page under AI Settings > Models, and the personal overrides
page gains an organization selector for users in multiple organizations.
Advisor telemetry now reports per-organization override rows.

**BREAKING CHANGE**: Existing deployment-level overrides are dropped,
not migrated. Every organization starts with no overrides after upgrade,
and model selections must be re-applied in the new settings. The legacy
deployment-level override endpoints are removed.

Closes https://linear.app/coder/issue/CODAGT-872

> [!NOTE]
> Xum (AI agent) authored this PR on behalf of @ibetitsmike.

<!-- xum-attribution: model=claude-opus-4-6 thinking=high -->

---------

Co-authored-by: Ethan Dickson <ethan@coder.com>

coderd/util/ptr/ptr.go

@@ -1,7 +1,11 @@
 // Package ptr contains some utility methods related to pointers.
 package ptr
 
-import "golang.org/x/exp/constraints"
+import (
+	"database/sql"
+
+	"golang.org/x/exp/constraints"
+)
 
 type number interface {
 	constraints.Integer | constraints.Float
@@ -38,3 +42,13 @@ func NilToDefault[T any](s *T, def T) T {
 func NilOrZero[T number](v *T) bool {
 	return v == nil || *v == 0
 }
+
+// FromNullString returns a pointer to the string when it is valid and nil
+// otherwise.
+func FromNullString(v sql.NullString) *string {
+	if !v.Valid {
+		return nil
+	}
+	value := v.String
+	return &value
+}

coderd/util/ptr/ptr_test.go

@@ -1,6 +1,7 @@
 package ptr_test
 
 import (
+	"database/sql"
 	"testing"
 	"time"
 
@@ -101,3 +102,18 @@ func Test_NilOrZero(t *testing.T) {
 	assert.False(t, ptr.NilOrZero(&nonZeroFloat64))
 	assert.False(t, ptr.NilOrZero(&nonZeroDuration))
 }
+
+func Test_FromNullString(t *testing.T) {
+	t.Parallel()
+
+	assert.Nil(t, ptr.FromNullString(sql.NullString{}))
+	assert.Nil(t, ptr.FromNullString(sql.NullString{String: "ignored"}))
+
+	valid := ptr.FromNullString(sql.NullString{String: "value", Valid: true})
+	assert.NotNil(t, valid)
+	assert.Equal(t, "value", *valid)
+
+	empty := ptr.FromNullString(sql.NullString{Valid: true})
+	assert.NotNil(t, empty)
+	assert.Equal(t, "", *empty)
+}