Embed a question and a stored note differently (Vikunja #371)

Note recall is asymmetric: a short question goes in, a longer note comes
out. Adds EmbedQuery/EmbedPassage helpers and the e5 prefixes, and points
the note/fact write path at the passage side and the query path at the
query side. Reviewers: the three call sites in voice.go.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/router/embedder.go

@@ -19,6 +19,37 @@ type Embedder interface {
 	Close() error
 }
 
+// AsymmetricEmbedder — an embedder that wants to know whether a text is a
+// search query or a stored passage. Recall is asymmetric: a short question
+// goes in, a longer note comes out. The e5 family is trained for exactly that
+// and needs the side written into the text ("query: " / "passage: ").
+//
+// Optional on purpose: HashEmbedder has no such notion, so callers go through
+// EmbedQuery and EmbedPassage below, which fall back to plain Embed.
+type AsymmetricEmbedder interface {
+	Embedder
+	EmbedQuery(ctx context.Context, text string) ([]float32, error)
+	EmbedPassage(ctx context.Context, text string) ([]float32, error)
+}
+
+// EmbedQuery embeds text that is being searched WITH — a question.
+func EmbedQuery(ctx context.Context, e Embedder, text string) ([]float32, error) {
+	if a, ok := e.(AsymmetricEmbedder); ok {
+		return a.EmbedQuery(ctx, text)
+	}
+	return e.Embed(ctx, text)
+}
+
+// EmbedPassage embeds text that is being searched FOR — a note or a fact on
+// its way into the store. Store and lookup must use these two calls, not one
+// of them twice, or the asymmetry buys nothing.
+func EmbedPassage(ctx context.Context, e Embedder, text string) ([]float32, error) {
+	if a, ok := e.(AsymmetricEmbedder); ok {
+		return a.EmbedPassage(ctx, text)
+	}
+	return e.Embed(ctx, text)
+}
+
 // HashEmbedder — a deterministic bag-of-words embedder used for tests and as a
 // non-zero default floor. NOT semantically meaningful across languages; the
 // real classifier swaps in the multilingual ONNX model wholesale.