Record which embedder wrote the stored vectors and warn on a swap (#378)

The embedder moved from paraphrase-multilingual-MiniLM-L12-v2 to
multilingual-e5-small. Both are 384-dimensional, so nothing in the code
noticed: cosine between an old stored vector and a new query vector is
noise, and recall degrades silently.

So the DB now records the embedder that wrote its vectors. One value for
the whole DB (migration #11, a small `meta` key/value table) rather than a
column on every vector row: the backfill re-embeds every note and fact in
one pass, so a per-row marker would hold the same string in every row and
cost a column on two tables for nothing.

The identity comes from the embedder itself via a new optional ID() method
("multilingual-e5-small@384", model file name plus dimension), so pointing
the config at another model changes the string without anyone editing a
constant. mavend logs a loud WARNING at startup naming both the stored and
the configured embedder when they differ.

Detection only — recall behaviour is unchanged. TODO(#378) in
store.CheckEmbedder marks where the backfill will hook in.

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

internal/router/embedder.go

@@ -2,6 +2,7 @@ package router
 
 import (
 	"context"
+	"fmt"
 	"math"
 	"unicode"
 )
@@ -19,6 +20,24 @@ type Embedder interface {
 	Close() error
 }
 
+// IdentifiedEmbedder — an embedder that can name itself. The name goes into
+// the DB next to the vectors it wrote, so a later model swap is caught instead
+// of silently returning nonsense scores (Vikunja #378).
+type IdentifiedEmbedder interface {
+	Embedder
+	ID() string
+}
+
+// EmbedderID is the stable string stored alongside the vectors. It comes from
+// the embedder itself — nobody hand-types a model name twice — and changes
+// whenever the model or its dimension changes.
+func EmbedderID(e Embedder) string {
+	if i, ok := e.(IdentifiedEmbedder); ok {
+		return i.ID()
+	}
+	return fmt.Sprintf("unknown@%d", e.Dim())
+}
+
 // 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
@@ -70,6 +89,10 @@ func NewHashEmbedder(dim int) *HashEmbedder {
 
 func (h *HashEmbedder) Dim() int { return h.dim }
 
+// ID names this embedder for the DB marker. The dimension is part of it
+// because a HashEmbedder of another width is a different vector space.
+func (h *HashEmbedder) ID() string { return fmt.Sprintf("hash@%d", h.dim) }
+
 func (h *HashEmbedder) Close() error { return nil }
 
 func (h *HashEmbedder) Embed(_ context.Context, text string) ([]float32, error) {

internal/router/embedderid_test.go

@@ -0,0 +1,24 @@
+package router
+
+import "testing"
+
+func TestEmbedderIDFromModelPath(t *testing.T) {
+	got := modelIDFromPath("/opt/maven/models/embedder/multilingual-e5-small.onnx")
+	if got != "multilingual-e5-small@384" {
+		t.Fatalf("modelIDFromPath = %q", got)
+	}
+	// A different model file must produce a different id, even at 384 dim.
+	old := modelIDFromPath("/opt/maven/models/embedder/paraphrase-multilingual-MiniLM-L12-v2.onnx")
+	if old == got {
+		t.Fatal("two different models share one id")
+	}
+}
+
+func TestEmbedderIDIncludesDim(t *testing.T) {
+	if id := EmbedderID(NewHashEmbedder(1024)); id != "hash@1024" {
+		t.Fatalf("EmbedderID = %q", id)
+	}
+	if EmbedderID(NewHashEmbedder(1024)) == EmbedderID(NewHashEmbedder(384)) {
+		t.Fatal("dimension not part of the id")
+	}
+}

internal/router/onnxembedder.go

@@ -33,6 +33,7 @@ const (
 type onnxEmbedder struct {
 	tokenizer *unigramTokenizer
 	session   *ort.DynamicSession[int64, float32]
+	id        string
 }
 
 func NewONNXEmbedder(modelPath, tokenizerPath, libPath string) (*onnxEmbedder, error) {
@@ -58,11 +59,31 @@ func NewONNXEmbedder(modelPath, tokenizerPath, libPath string) (*onnxEmbedder, e
 	return &onnxEmbedder{
 		tokenizer: tok,
 		session:   session,
+		id:        modelIDFromPath(modelPath),
 	}, nil
 }
 
 func (e *onnxEmbedder) Dim() int { return embedDim }
 
+// ID names the loaded model for the DB marker (Vikunja #378): the model file's
+// own name plus the dimension, so pointing the config at another model changes
+// the string on its own.
+func (e *onnxEmbedder) ID() string { return e.id }
+
+// modelIDFromPath turns /opt/.../multilingual-e5-small.onnx into
+// "multilingual-e5-small@384".
+func modelIDFromPath(modelPath string) string {
+	name := modelPath
+	if i := strings.LastIndexAny(name, "/\\"); i >= 0 {
+		name = name[i+1:]
+	}
+	name = strings.TrimSuffix(name, ".onnx")
+	if name == "" {
+		name = "onnx"
+	}
+	return fmt.Sprintf("%s@%d", name, embedDim)
+}
+
 // Embed treats the text as a query. The classifier compares one short
 // utterance to another short seed phrase, so both sides get the same prefix
 // and the comparison stays fair. The recall path must call EmbedQuery and