feat(mcp): forbind med kontoen i stedet for med et token

Claude's forbindelsesdialog prøver OAuth først. Indtil nu var svaret 404,
og eneste vej ind var at lave et token og sætte det ind i en adresse. Nu
er instansen sin egen autorisationsserver for MCP-klienter:

- /.well-known/oauth-protected-resource(/mcp) og
  /.well-known/oauth-authorization-server peger på login'et; en 401 fra
  /mcp bærer WWW-Authenticate med resource_metadata
- /oauth/register (RFC 7591), kun offentlige klienter; registreringer,
  ingen godkender, ryddes efter et døgn
- /oauth/authorize sender til /forbind, som logger ind med det almindelige
  login og spørger "Claude vil forbinde til din konto" — og viser, hvor
  adgangen sendes hen, fordi navnet er klientens eget
- /oauth/token: authorization code + S256 PKCE, koden gælder ét minut og
  én gang; ud kommer et almindeligt API-token, listet under Tokens med
  klientens navn, og en ny forbindelse fra samme klient erstatter det
- /mcp tager nu også Authorization: Bearer; ?key= virker stadig

Co-Authored-By: Claude Opus 5.5 <[email protected]>

internal/store/oauth.go

@@ -0,0 +1,99 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"errors"
+	"time"
+
+	"github.com/kristianwind/verdande/internal/auth"
+)
+
+// OAuthClient is a program that registered itself to connect to an account: in
+// practice an MCP client such as Claude. It holds no secret — see migration 0038.
+type OAuthClient struct {
+	ID           string
+	Name         string
+	RedirectURIs []string
+	CreatedAt    time.Time
+}
+
+// unusedClientAge is how long a registration may sit without ever being
+// approved before it is cleared. Registration is open to anybody who can reach
+// the instance, so without this the table grows with every abandoned attempt.
+const unusedClientAge = 24 * time.Hour
+
+func (db *DB) CreateOAuthClient(ctx context.Context, name string, redirectURIs []string) (*OAuthClient, error) {
+	uris, err := json.Marshal(redirectURIs)
+	if err != nil {
+		return nil, err
+	}
+	c := &OAuthClient{ID: NewID(), Name: name, RedirectURIs: redirectURIs, CreatedAt: time.Now().UTC()}
+
+	// Swept on the way in, like the socket tickets: the table is bounded by a
+	// day of asking, and nothing has to remember to run a job for it.
+	_, _ = db.ExecContext(ctx,
+		`DELETE FROM oauth_clients WHERE created_at < ?
+		 AND NOT EXISTS (SELECT 1 FROM api_tokens WHERE oauth_client_id = oauth_clients.id)`,
+		time.Now().Add(-unusedClientAge).Unix())
+
+	_, err = db.ExecContext(ctx,
+		`INSERT INTO oauth_clients (id, name, redirect_uris, created_at) VALUES (?, ?, ?, ?)`,
+		c.ID, c.Name, string(uris), c.CreatedAt.Unix())
+	if err != nil {
+		return nil, err
+	}
+	return c, nil
+}
+
+func (db *DB) OAuthClient(ctx context.Context, id string) (*OAuthClient, error) {
+	var c OAuthClient
+	var uris string
+	var created int64
+	err := db.QueryRowContext(ctx,
+		`SELECT id, name, redirect_uris, created_at FROM oauth_clients WHERE id = ?`, id).
+		Scan(&c.ID, &c.Name, &uris, &created)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	if err := json.Unmarshal([]byte(uris), &c.RedirectURIs); err != nil {
+		return nil, err
+	}
+	c.CreatedAt = time.Unix(created, 0).UTC()
+	return &c, nil
+}
+
+// CreateOAuthToken mints the API token an approval hands to a client.
+//
+// Any earlier token this person gave the same client is deleted in the same
+// transaction. A client that connects again — after a reinstall, or because
+// somebody pressed "connect" twice — then has one entry on the tokens page
+// rather than a growing column of identical ones, each still valid.
+func (db *DB) CreateOAuthToken(ctx context.Context, userID string, client *OAuthClient) (string, error) {
+	token, prefix, err := auth.NewAPIToken()
+	if err != nil {
+		return "", err
+	}
+
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return "", err
+	}
+	defer tx.Rollback()
+
+	if _, err := tx.ExecContext(ctx,
+		`DELETE FROM api_tokens WHERE user_id = ? AND oauth_client_id = ?`, userID, client.ID); err != nil {
+		return "", err
+	}
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO api_tokens (id, user_id, name, token_hash, prefix, created_at, oauth_client_id)
+		 VALUES (?, ?, ?, ?, ?, ?, ?)`,
+		NewID(), userID, client.Name, auth.HashToken(token), prefix, time.Now().Unix(), client.ID); err != nil {
+		return "", err
+	}
+	return token, tx.Commit()
+}