Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ROADMAP.md

@@ -0,0 +1,11 @@
+# Roadmap
+
+Implementation strategy and record of existing/planned features. Completed items are
+terse checklists - technical detail lives in the architecture docs (`docs-private/architecture/`). Remaining tasks keep
+enough scope to be actionable.
+
+Legend: `[x]` done · `[ ]` planned · sub-lists split done vs. remaining for partial items.
+
+Within each phase, remaining `[ ]` items are listed in rough priority order: correctness
+and robustness of the existing modeling workflow (especially after edits/imports) precede
+new feature breadth.