ci: add workflow_dispatch to enable manual CI runs on stack branches (#2559)

After PR #2558 removed `spr/**` from the `pull_request.branches` filter in
the three Delphi CI workflows, intermediate stack PRs (whose base is
`spr/edge/<sha>`) no longer auto-trigger CI on every rebase — only the
bottom PR (base = `edge`) does. That solved the CI-overload problem from
spr's force-push cascades but left no way to manually validate an
intermediate PR before it became the bottom of the stack.

Adding `workflow_dispatch` gives repo write-access users an opt-in manual
trigger:
- **UI**: Actions → workflow → "Run workflow" dropdown → pick branch → Run.
- **CLI**: `gh workflow run "Delphi Python Tests" --ref spr/edge/<sha>`.

Security note: `workflow_dispatch` is restricted by GitHub to users with
`write` permission on the repo. Fork-PR authors cannot invoke it (403),
so this does not weaken the existing fork-PR safeguards (which govern
`pull_request` events, a separate concern).

Workflows updated:
- `.github/workflows/python-ci.yml` (Delphi Python Tests)
- `.github/workflows/cypress-tests.yml` (E2E Tests)
- `.github/workflows/jest-server-test.yml` (Server Integration Tests)

The `lint.yml` workflow already runs unconditionally on every PR via its
`types: [opened, reopened, synchronize]` filter, so it doesn't need this.

Author: jucor

.github/workflows/cypress-tests.yml

@@ -8,6 +8,11 @@ on:
       - edge
       - stable
       - 'jc/**'
+  # Manual trigger: lets repo write-access users run CI on any branch
+  # (including spr/edge/* stack branches, which `pull_request` no longer
+  # auto-triggers for). UI: Actions → E2E Tests → "Run workflow".
+  # CLI: `gh workflow run "E2E Tests" --ref <branch>`.
+  workflow_dispatch:
 
 concurrency:
   group: e2e-${{ github.ref }}

.github/workflows/jest-server-test.yml

@@ -8,6 +8,11 @@ on:
       - edge
       - stable
       - 'jc/**'
+  # Manual trigger: lets repo write-access users run CI on any branch
+  # (including spr/edge/* stack branches, which `pull_request` no longer
+  # auto-triggers for). UI: Actions → Server Integration Tests → "Run workflow".
+  # CLI: `gh workflow run "Server Integration Tests" --ref <branch>`.
+  workflow_dispatch:
 
 jobs:
   server-integration-tests:

.github/workflows/python-ci.yml

@@ -14,6 +14,11 @@ on:
       - stable
       - 'jc/**'
     paths: *paths
+  # Manual trigger: lets repo write-access users run CI on any branch
+  # (including spr/edge/* stack branches, which `pull_request` no longer
+  # auto-triggers for). UI: Actions → Delphi Python Tests → "Run workflow".
+  # CLI: `gh workflow run "Delphi Python Tests" --ref <branch>`.
+  workflow_dispatch:
 
 jobs:
   test: