Fix three defects a full VM migration exposed

Ran a complete 0.15.5 -> 0.16.14 migration of the smoke VM, driving the
phases in the order the real pipeline will. It worked - all mail intact and
readable afterwards, all ten listeners up, cutover executed for the first
time ever and checkpoint resume exercised - and it exposed three defects.

1. The converted config was installed root-owned while the service runs as
   its own user. Stalwart crash-looped 28 times on "Failed to read data
   store settings: Permission denied", minutes after the mistake and
   nowhere near it. This is the same ownership trap that retired the
   rollback implementation, in a new place: writing files as root is the
   natural thing for a tool running as root to do, and it is wrong every
   time the service is not root.

   Cutover now installs the config itself, copying ownership and mode from
   the config being replaced.

2. v0.16.14 does not serve /api - the endpoint stalwartapi assumed.
   Confirmed against a fully migrated, fully configured, serving instance
   rather than a sandbox: /api, /api/principal and /jmap/ all 404. The JMAP
   endpoint is the one the session document advertises, which is what RFC
   8620 discovery is for.

   The client now discovers it, re-basing the advertised path onto the
   operator's host: a real instance advertises its canonical public URL
   ("https://mail.smoke.test/jmap/") which frequently isn't reachable from
   where this tool runs. The session is authoritative about the path; the
   operator is authoritative about the host.

3. Dispatching on the urn:stalwart:jmap capability was wrong, because
   NEITHER version advertises it - not 0.15.5, and not a fully migrated
   0.16.14. That sent 0.16 instances down the 0.15 REST path where every
   call 404s. The client probes what the instance actually serves instead.
   Less elegant than a declared capability, with the advantage of being
   true.

Also: a JMAP "forbidden" now explains itself. An account holding the admin
role before the migration was refused x:Account/query afterwards, and a
bare "forbidden" gives an operator nowhere to start. Whether the role
failed to carry or v0.16 wants different permissions was not isolated, and
that question is recorded as open - it gates quota recalculation and any
post-migration validation.

Verified against both live instances: the 0.15.5 reports 3 accounts and its
domain over REST, and the migrated 0.16.14 routes to JMAP, finds the right
endpoint, and returns the explained refusal.

internal/stalwartapi/client.go

@@ -7,7 +7,9 @@ import (
 	"context"
 	"fmt"
 	"net/http"
+	"net/url"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -24,6 +26,66 @@ type Client struct {
 	Username   string
 	Password   string
 	HTTPClient *http.Client
+
+	mu       sync.Mutex
+	endpoint string // resolved JMAP endpoint; see apiEndpoint
+}
+
+// apiEndpoint returns the URL to POST JMAP method calls to, discovered from
+// the instance's own session document rather than assumed.
+//
+// This used to be hardcoded as BaseURL + "/api", which is wrong for the
+// version this tool migrates *to*: a fully configured, serving 0.16.14
+// returns 404 for /api, and advertises its JMAP endpoint through the
+// session document's apiUrl instead (RFC 8620 §2 - discovery is how a
+// client is *supposed* to find it).
+//
+// The path is taken from apiUrl but re-based onto BaseURL's scheme and
+// host. A real instance advertises its canonical public URL - observed:
+// "https://mail.smoke.test/jmap/" - which frequently isn't reachable from
+// where this tool runs, over a hostname that may not resolve or a
+// certificate that may not validate. The operator told us how to reach
+// this server when they passed --admin-url; the session is only authoritative
+// about *where on it* the API lives.
+func (c *Client) apiEndpoint(ctx context.Context) (string, error) {
+	c.mu.Lock()
+	cached := c.endpoint
+	c.mu.Unlock()
+	if cached != "" {
+		return cached, nil
+	}
+
+	session, err := c.fetchSession(ctx, c.Username, c.Password)
+	if err != nil {
+		return "", fmt.Errorf("stalwartapi: discover the JMAP endpoint: %w", err)
+	}
+	resolved, err := c.rebaseOntoBaseURL(session.APIURL)
+	if err != nil {
+		return "", err
+	}
+
+	c.mu.Lock()
+	c.endpoint = resolved
+	c.mu.Unlock()
+	return resolved, nil
+}
+
+// rebaseOntoBaseURL keeps the advertised path but the operator's host.
+func (c *Client) rebaseOntoBaseURL(apiURL string) (string, error) {
+	base, err := url.Parse(strings.TrimRight(c.BaseURL, "/"))
+	if err != nil {
+		return "", fmt.Errorf("stalwartapi: parse base URL %q: %w", c.BaseURL, err)
+	}
+	if apiURL == "" {
+		return "", fmt.Errorf("stalwartapi: the instance's session document advertises no apiUrl, so there is no JMAP endpoint to call")
+	}
+	advertised, err := url.Parse(apiURL)
+	if err != nil {
+		return "", fmt.Errorf("stalwartapi: parse advertised apiUrl %q: %w", apiURL, err)
+	}
+	base.Path = advertised.Path
+	base.RawQuery = advertised.RawQuery
+	return base.String(), nil
 }
 
 func (c *Client) httpClient() *http.Client {