chore(deps): update Go modules

Bump Go dependencies in both backend/ and backend/_example/memory_store.

Notable updates:
- github.com/go-pkgz/lgr v0.12.1 -> v0.12.3
- github.com/klauspost/compress v1.18.2 -> v1.18.5
- github.com/PuerkitoBio/goquery v1.11.0 -> v1.12.0
- github.com/montanaflynn/stats v0.7.1 -> v0.9.0
- github.com/redis/go-redis/v9 v9.17.2 -> v9.18.0
- github.com/slack-go/slack v0.17.3 -> v0.21.1
- go.mongodb.org/mongo-driver v1.17.6 -> v1.17.9
- golang.org/x/crypto v0.48.0 -> v0.50.0
- golang.org/x/net v0.49.0 -> v0.53.0
- golang.org/x/image v0.36.0 -> v0.39.0
- golang.org/x/sys v0.41.0 -> v0.43.0
- golang.org/x/{oauth2,sync,text} minor bumps

Key markdown/sanitisation libs (bluemonday v1.0.27,
alecthomas/chroma/v2 v2.23.1, russross/blackfriday/v2 v2.1.0,
Depado/bfchroma/v2 v2.0.0) are already at the latest available
versions and were not bumped.

Verified the Chroma span-class allowlist regex in
backend/app/store/comment.go:128-131 is still fully in sync with
chroma/v2 types.go StandardTypes map (86 classes, byte-equal after
sorting). The inline comment references commit c263f6f which is
stale (Chroma is at v2 now), but the class list content is current.

Ran `go mod tidy` + `go mod vendor` + full race test suite on both
modules. All green. Added a reminder in CLAUDE.md that updating
backend/ Go modules also requires `go mod tidy` in
backend/_example/memory_store since the example module uses a
local replace directive and inherits indirect deps from the main
module.

backend/vendor/github.com/slack-go/slack/function_execute.go

@@ -43,14 +43,13 @@ func (api *Client) FunctionCompleteSuccessContext(ctx context.Context, functionE
 		option(r)
 	}
 
-	endpoint := api.endpoint + "functions.completeSuccess"
 	jsonData, err := json.Marshal(r)
 	if err != nil {
 		return err
 	}
 
 	response := &SlackResponse{}
-	if err := postJSON(ctx, api.httpclient, endpoint, api.token, jsonData, response, api); err != nil {
+	if err := api.postJSONMethod(ctx, "functions.completeSuccess", api.token, jsonData, response); err != nil {
 		return err
 	}
 
@@ -74,14 +73,13 @@ func (api *Client) FunctionCompleteErrorContext(ctx context.Context, functionExe
 	}
 	r.Error = errorMessage
 
-	endpoint := api.endpoint + "functions.completeError"
 	jsonData, err := json.Marshal(r)
 	if err != nil {
 		return err
 	}
 
 	response := &SlackResponse{}
-	if err := postJSON(ctx, api.httpclient, endpoint, api.token, jsonData, response, api); err != nil {
+	if err := api.postJSONMethod(ctx, "functions.completeError", api.token, jsonData, response); err != nil {
 		return err
 	}