feat: add user AI spend endpoint (#26978)

## Description

Adds the `GET /api/v2/users/{user}/ai/spend` endpoint returning the
user's current AI spend, effective budget, and period bounds.

## Changes

- Add `userAISpendStatus` handler under the same feature/experiment gate
as `/api/v2/users/{user}/ai/budget`.
- Add `codersdk.UserAIBudgetSummary` (embedded into `UserAISpendStatus`)
and a `UserAISpendStatus` client method.
- Move `LimitSource` from `coderd/aibridge/budget` to `codersdk` so the
type is shared across endpoints.

Closes https://linear.app/codercom/issue/AIGOV-472

> [!NOTE]
> Initially generated by Claude Opus 4.7, modified and reviewed by
@ssncferreira

coderd/apidoc/docs.go

@@ -9652,6 +9652,40 @@ const docTemplate = `{
                 ]
             }
         },
+        "/api/v2/users/{user}/ai/spend": {
+            "get": {
+                "produces": [
+                    "application/json"
+                ],
+                "tags": [
+                    "Enterprise"
+                ],
+                "summary": "Get user AI spend",
+                "operationId": "get-user-ai-spend",
+                "parameters": [
+                    {
+                        "type": "string",
+                        "description": "User ID, username, or me",
+                        "name": "user",
+                        "in": "path",
+                        "required": true
+                    }
+                ],
+                "responses": {
+                    "200": {
+                        "description": "OK",
+                        "schema": {
+                            "$ref": "#/definitions/codersdk.UserAISpendStatus"
+                        }
+                    }
+                },
+                "security": [
+                    {
+                        "CoderSessionToken": []
+                    }
+                ]
+            }
+        },
         "/api/v2/users/{user}/appearance": {
             "get": {
                 "produces": [
@@ -15251,6 +15285,17 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.AIBudgetLimitSource": {
+            "type": "string",
+            "enum": [
+                "user_override",
+                "group"
+            ],
+            "x-enum-varnames": [
+                "AIBudgetLimitSourceUserOverride",
+                "AIBudgetLimitSourceGroup"
+            ]
+        },
         "codersdk.AIConfig": {
             "type": "object",
             "properties": {
@@ -25657,6 +25702,46 @@ const docTemplate = `{
                 }
             }
         },
+        "codersdk.UserAISpendStatus": {
+            "type": "object",
+            "properties": {
+                "current_spend_micros": {
+                    "description": "CurrentSpendMicros is the user's spend on their effective group over\nthe current budget period.",
+                    "type": "integer"
+                },
+                "effective_group_id": {
+                    "description": "EffectiveGroupID is the group the spend is attributed to. Null when\nno budget applies.",
+                    "type": "string",
+                    "format": "uuid"
+                },
+                "limit_source": {
+                    "description": "LimitSource identifies which tier produced the limit. Null when no\nbudget applies.",
+                    "allOf": [
+                        {
+                            "$ref": "#/definitions/codersdk.AIBudgetLimitSource"
+                        }
+                    ]
+                },
+                "period_end": {
+                    "description": "PeriodEnd is the exclusive upper bound of the current budget\nperiod.",
+                    "type": "string",
+                    "format": "date-time"
+                },
+                "period_start": {
+                    "description": "PeriodStart is the inclusive lower bound of the current budget\nperiod.",
+                    "type": "string",
+                    "format": "date-time"
+                },
+                "spend_limit_micros": {
+                    "description": "SpendLimitMicros is the effective spend limit in micro-units.\nNull when no budget applies to the user (unlimited).",
+                    "type": "integer"
+                },
+                "user_id": {
+                    "type": "string",
+                    "format": "uuid"
+                }
+            }
+        },
         "codersdk.UserActivity": {
             "type": "object",
             "properties": {