chore(deps): consolidate dev deps into [project.optional-dependencies] dev

Follow-up to 7c1be58. pytest-randomly was added to a PEP 735
[dependency-groups] dev block by 'uv add --dev', leaving the
project with two 'dev' groups (the legacy [project.optional-
dependencies] dev and the new [dependency-groups] dev). To get
the full dev env someone had to run 'uv sync --extra dev --group
dev', which is easy to miss.

This commit moves pytest-randomly>=4.1 into the
[project.optional-dependencies] dev list (where pytest,
pytest-cov, pytest-asyncio, and httpx already live) and drops
the [dependency-groups] block. 'uv sync --extra dev' now
installs the full dev toolchain. uv.lock updated accordingly:
the [package.dev-dependencies] and [package.metadata.requires-
dev] sections are gone, and pytest-randomly is part of the
standard dev extra.

backend/pyproject.toml

@@ -29,6 +29,7 @@ dev = [
     "pytest-cov>=4.1",
     "pytest-asyncio>=0.23,<1",
     "httpx>=0.27,<1",
+    "pytest-randomly>=4.1",
 ]
 sqlcipher = [
     # SP12: encryption at rest. Optional — without it the DB is plain SQLite.
@@ -51,8 +52,3 @@ where = ["src"]
 testpaths = ["tests"]
 addopts = "-ra --strict-markers"
 asyncio_mode = "auto"
-
-[dependency-groups]
-dev = [
-    "pytest-randomly>=4.1.0",
-]