fix(limits): lower the session ceiling to 462 so the layout budget holds

An owner's tab layout holds at most MAX_TAB_REFS (512) refs, counting saved web
tabs (up to MAX_WEBVIEWS, 50 by default) as well as sessions. With a ceiling of
500, session creation could still fail with HTTP 500 on the layout limit before
the capacity message. 512 - 50 = 462; a test pins the ceiling plus MAX_WEBVIEWS
inside MAX_TAB_REFS so the two limits cannot drift apart again.

Co-Authored-By: Christian Haberl <116439789+christianhaberl@users.noreply.github.com>
Co-Authored-By: Claude <noreply@anthropic.com>

docs/wiki/HTTP-API.md

@@ -185,7 +185,7 @@ curl -s -X POST "$API/api/sessions/$ID/custom-model" -H 'Content-Type: applicati
 
 | Limit                 | Default                                    |
 | --------------------- | ------------------------------------------ |
-| Max sessions          | 50                                          |
+| Max sessions          | 50 (`CODEMAN_MAX_SESSIONS`)                 |
 | Max agent windows     | 500                                         |
 | Max SSE clients       | 100                                         |
 | Terminal buffer       | 32 MB per session                           |

docs/wiki/Settings-Reference.md

@@ -221,7 +221,7 @@ Some things are configured before the server starts, not in the UI:
 | `CODEMAN_FILE_PICKER_ROOTS`         | Extra roots for the path picker.                                        |
 | `CODEMAN_ALLOW_UNAUTHENTICATED_NETWORK` | Acknowledges exposing the server with no password.                  |
 | `CODEMAN_BASE_URL`                  | Mounts Codeman under a sub-path behind a reverse proxy that forwards the prefix unchanged. See [Remote Access](Remote-Access). |
-| `CODEMAN_MAX_SESSIONS`              | Cap on concurrent sessions. 50 by default, 1 to 500 (a larger value is clamped to 500, anything that is not a positive integer falls back to 50). Read at startup. In multi-user mode the per-user cap defaults to half of it unless `CODEMAN_MAX_SESSIONS_PER_USER` is set. |
+| `CODEMAN_MAX_SESSIONS`              | Cap on concurrent sessions. 50 by default, 1 to 462 (a larger value is clamped to 462, anything that is not a positive integer falls back to 50). Read at startup. In multi-user mode the per-user cap defaults to half of it unless `CODEMAN_MAX_SESSIONS_PER_USER` is set. |
 | `CODEMAN_MAX_DOWNLOAD_BYTES`        | Cap on raw file bodies and downloads. 2 GB by default, `0` for none.    |
 | `CODEMAN_MAX_PASTE_IMAGE_BYTES`     | Cap on one pasted, dropped or attached image or document. 50 MB by default. |
 | `CODEMAN_MAX_PASTE_VIDEO_BYTES`     | Cap on one attached video (phone Attach, drop). 1 GB by default; videos stream to disk, so this bounds disk use, not memory. |