Plumb ctx through classifier and Ollama HTTP path

OllamaClient.Generate/GenerateText/call now take ctx and build the HTTP
request via NewRequestWithContext, so an in-flight LLM call is aborted
when the parent context is cancelled (Ctrl-C). Classifier.Classify and
its tier helpers take ctx too. ProcessFunc gets a ctx parameter so the
poller can forward its cancellable context down to classification.

Explainer.summarize manages its own 60s context since reaction-driven
flow has no parent ctx to inherit.

internal/explainer/explainer.go

@@ -4,6 +4,7 @@
 package explainer
 
 import (
+	"context"
 	"fmt"
 	"html"
 	"log/slog"
@@ -125,8 +126,11 @@ Rules:
 - Stick strictly to facts in the article. Do not speculate.`
 
 func (e *Explainer) summarize(headline, body string) (string, error) {
+	// Reaction-driven flow has no parent ctx; bound the LLM call ourselves.
+	ctx, cancel := context.WithTimeout(context.Background(), 60*time.Second)
+	defer cancel()
 	prompt := fmt.Sprintf("Headline: %s\n\nArticle body:\n%s", headline, body)
-	out, err := e.ollama.GenerateText(summarySystem, prompt)
+	out, err := e.ollama.GenerateText(ctx, summarySystem, prompt)
 	if err != nil {
 		return "", err
 	}