feat(phase5): MCP server, CalDAV, mail-to-task, Gmail settings and an AI layer

The MCP server speaks JSON-RPC over Streamable HTTP with the six tools
the brief names, authenticated by a personal API token — so a connector
reaches exactly what its owner reaches and there is no second identity
to reason about. A failed tool call returns a tool error, not a protocol
error: a model can recover from the first and can do nothing with the
second.

CalDAV serves VTODO, two-way, so Apple Reminders and Thunderbird can
read, create and complete tasks. Clients authenticate with Basic using
an API token as the password, never the account password — a client
stores what it is given, and what it is given should not be able to sign
in to the app. Completed tasks stay in the collection: a client that
stops seeing one treats it as deleted and drops its own record of having
done it.

The AI layer is one interface with adapters for Anthropic, OpenAI,
Google and any OpenAI-compatible endpoint. That last one is why the
abstraction exists: an integration that only spoke to a hosted API would
be useless to somebody running their own models. Every feature degrades
to "off" when nothing is configured, and the stored API key is never
returned to the settings page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/notifications.go

@@ -3,7 +3,10 @@ package store
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"time"
+
+	"github.com/kristianwind/verdande/internal/auth"
 )
 
 type Notification struct {
@@ -230,3 +233,78 @@ func (db *DB) InstanceKeys(ctx context.Context, generate func() (public, private
 	}
 	return public, private, nil
 }
+
+// --- mail-to-task tokens -------------------------------------------------------------
+
+func (db *DB) EnsureMailToken(ctx context.Context, userID string) (string, error) {
+	var token sql.NullString
+	err := db.QueryRowContext(ctx, `SELECT mail_token FROM users WHERE id = ?`, userID).Scan(&token)
+	if err != nil {
+		return "", err
+	}
+	if token.Valid && token.String != "" {
+		return token.String, nil
+	}
+
+	fresh, err := auth.NewToken()
+	if err != nil {
+		return "", err
+	}
+	if err := db.SetMailToken(ctx, userID, fresh); err != nil {
+		return "", err
+	}
+	return fresh, nil
+}
+
+func (db *DB) SetMailToken(ctx context.Context, userID, token string) error {
+	_, err := db.ExecContext(ctx,
+		`UPDATE users SET mail_token = ?, updated_at = ? WHERE id = ?`,
+		token, time.Now().Unix(), userID)
+	return err
+}
+
+func (db *DB) UserByMailToken(ctx context.Context, token string) (*User, error) {
+	if token == "" {
+		return nil, ErrNotFound
+	}
+	return db.scanUser(ctx, `SELECT `+userColumns+` FROM users WHERE mail_token = ?`, token)
+}
+
+// --- per-user settings ------------------------------------------------------------------
+
+// UserSettings reads one scope of a person's integration settings. A missing row is
+// an empty map, not an error: nobody has settings until they set some.
+func (db *DB) UserSettings(ctx context.Context, userID, scope string) (map[string]any, error) {
+	var raw string
+	err := db.QueryRowContext(ctx,
+		`SELECT values_json FROM user_settings WHERE user_id = ? AND scope = ?`,
+		userID, scope).Scan(&raw)
+	if err == sql.ErrNoRows {
+		return map[string]any{}, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	values := map[string]any{}
+	if err := json.Unmarshal([]byte(raw), &values); err != nil {
+		return map[string]any{}, nil
+	}
+	return values, nil
+}
+
+func (db *DB) SetUserSettings(ctx context.Context, userID, scope string, values map[string]any) error {
+	raw, err := json.Marshal(values)
+	if err != nil {
+		return err
+	}
+	_, err = db.ExecContext(ctx, `
+		INSERT INTO user_settings (user_id, scope, values_json, updated_at)
+		VALUES (?, ?, ?, ?)
+		ON CONFLICT (user_id) DO UPDATE SET
+		    scope = excluded.scope,
+		    values_json = excluded.values_json,
+		    updated_at = excluded.updated_at`,
+		userID, scope, string(raw), time.Now().Unix())
+	return err
+}