feat: add labels to chats (#23594)

## Summary

Adds a general-purpose `map[string]string` label system to chats, stored
as jsonb with a GIN index for efficient containment queries.

This is a standalone foundational feature that will be used by the
upcoming Automations feature for session identity (matching webhook
events to existing chats), replacing the need for bespoke session-key
tables.

## Changes

### Database
- **Migration 000451**: Adds `labels jsonb NOT NULL DEFAULT '{}'` column
to `chats` table with a GIN index (`idx_chats_labels`)
- **`InsertChat`**: Accepts labels on creation via `COALESCE(@labels,
'{}')`
- **`UpdateChatByID`**: Supports partial update —
`COALESCE(sqlc.narg('labels'), labels)` preserves existing labels when
NULL is passed
- **`GetChats`**: New `has_labels` filter using PostgreSQL `@>`
containment operator
- **`GetAuthorizedChats`**: Synced with generated `GetChats` (new column
scan + query param)

### API
- **Create chat** (`POST /chats`): Accepts optional `labels` field,
validated before creation
- **Update chat** (`PATCH /chats/{chat}`): Supports `labels` field for
atomic label replacement
- **List chats** (`GET /chats`): Supports `?label=key:value` query
parameters (multiple are AND-ed)

### SDK
- `Chat`, `CreateChatRequest`, `UpdateChatRequest`, `ListChatsOptions`
all gain `Labels` fields
- `UpdateChatRequest.Labels` is a pointer (`*map[string]string`) so
`nil` means "don't change" vs empty map means "clear all"

### Validation (`coderd/httpapi/labels.go`)
- Max 50 labels per chat
- Key: 1–64 chars, must match `[a-zA-Z0-9][a-zA-Z0-9._/-]*` (supports
namespaced keys like `github.repo`, `automation/pr-number`)
- Value: 1–256 chars
- 13 test cases covering all edge cases

### Chat runtime
- `chatd.CreateOptions` gains `Labels` field, threaded through to
`InsertChat`
- Existing `UpdateChatByID` callers (e.g., quickgen title updates) are
unaffected — NULL labels preserve existing values via COALESCE

coderd/database/querier.go

@@ -823,6 +823,7 @@ type sqlcQuerier interface {
 	// Bumps the heartbeat timestamp for a running chat so that other
 	// replicas know the worker is still alive.
 	UpdateChatHeartbeat(ctx context.Context, arg UpdateChatHeartbeatParams) (int64, error)
+	UpdateChatLabelsByID(ctx context.Context, arg UpdateChatLabelsByIDParams) (Chat, error)
 	UpdateChatMCPServerIDs(ctx context.Context, arg UpdateChatMCPServerIDsParams) (Chat, error)
 	UpdateChatMessageByID(ctx context.Context, arg UpdateChatMessageByIDParams) (ChatMessage, error)
 	UpdateChatModelConfig(ctx context.Context, arg UpdateChatModelConfigParams) (ChatModelConfig, error)