chore(signup): bump wisp/mist to latest, lock with manifest.toml

Two prior deploy failures came from transitive deps drifting on every
fresh build (gleam_httpc 4.1.1 calling result.then, then glisten 8.0.3
calling list.range). The root cause is no manifest.toml — every build
re-resolved deps from scratch and tripped on broken combinations within
the allowed ranges.

- Bump wisp 1.x → 2.x and mist 4.x → 6.x so the resolver picks
  modern, mutually compatible versions of the whole tree
  (stdlib 1.0.3, glisten 9.0.1, httpc 5.0.0).
- Commit manifest.toml so deps are pinned by checksum.
- Adapt to wisp 2.x: json_response now takes String, not StringTree.
- Drop unused gleam/http/response import.
- Add signup/build/ to .gitignore.

signup/src/coolcouch_signup.gleam

@@ -4,7 +4,6 @@ import gleam/dynamic/decode
 import gleam/erlang/process
 import gleam/http.{Get, Options, Post}
 import gleam/http/request
-import gleam/http/response
 import gleam/httpc
 import gleam/int
 import gleam/io
@@ -231,5 +230,5 @@ fn error_json(status: Int, code: String) -> Response {
 }
 
 fn json_response(status: Int, body: json.Json) -> Response {
-  wisp.json_response(json.to_string_tree(body), status)
+  wisp.json_response(json.to_string(body), status)
 }