test(ttp): E.2.13 repository tests — TTPMixin idempotency + identity-rollup projection on dual backends

Adds tests/web/db/conftest.py with a db_backends fixture
parametrizing SQLite (always) + MySQL (gated on
DECNET_TEST_MYSQL_URL). Surface assertions (mixin methods present
+ async) GREEN today; insert_tags idempotency, identity rollup
projection, attacker-rollup exclusion of NULL-attacker tags
xfail-gated behind E.3.3.

tests/web/db/conftest.py

@@ -0,0 +1,71 @@
+"""Shared fixtures for ``tests/web/db/`` — dual-backend repo testing.
+
+The ``db_backends`` fixture parametrizes a repository instance over
+SQLite (always) and MySQL (skipped when ``DECNET_TEST_MYSQL_URL`` is
+unset). This is the single source of truth referenced by the design
+doc's "every repo test runs against both SQLite and MySQL"
+convention; new repo tests under ``tests/web/db/`` should consume
+the fixture rather than instantiating their own backend.
+
+MySQL is gated on env var rather than auto-detected because spinning
+a real MySQL is heavy enough to belong in CI / live runs but not the
+dev loop. Per project memory: "skip heavy test categories" in the
+dev cycle.
+"""
+from __future__ import annotations
+
+import os
+from pathlib import Path
+from typing import AsyncIterator
+
+import pytest
+import pytest_asyncio
+
+from decnet.web.db.factory import get_repository
+from decnet.web.db.repository import BaseRepository
+
+
+_BACKENDS: list[str] = ["sqlite"]
+if os.environ.get("DECNET_TEST_MYSQL_URL"):
+    _BACKENDS.append("mysql")
+
+
+@pytest_asyncio.fixture(params=_BACKENDS, ids=_BACKENDS)
+async def db_backends(
+    request: pytest.FixtureRequest,
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> AsyncIterator[BaseRepository]:
+    """Yield an initialized :class:`BaseRepository` for each available
+    backend.
+
+    SQLite always runs (via ``aiosqlite`` + a tmp file). MySQL runs
+    iff ``DECNET_TEST_MYSQL_URL`` is set in the environment to a real
+    MySQL DSN — in that case the fixture writes a per-test schema
+    name into ``DECNET_DB_URL`` so concurrent tests don't collide.
+    """
+    backend = request.param
+    if backend == "sqlite":
+        monkeypatch.setenv("DECNET_DB_TYPE", "sqlite")
+        repo = get_repository(db_path=str(tmp_path / "ttp.db"))
+    else:
+        # MySQL — uses the operator-supplied DSN. Per dual-DB-backend
+        # convention, dialect-specific behavior overrides land in the
+        # MySQL repo class; this fixture does not paper over them.
+        url = os.environ["DECNET_TEST_MYSQL_URL"]
+        monkeypatch.setenv("DECNET_DB_TYPE", "mysql")
+        monkeypatch.setenv("DECNET_DB_URL", url)
+        repo = get_repository()
+    await repo.initialize()
+    try:
+        yield repo
+    finally:
+        # SQLite is fully isolated per tmp_path; MySQL needs explicit
+        # teardown that's the operator's responsibility (truncate or
+        # drop schema in a CI hook). The repo close is best-effort.
+        engine = getattr(repo, "engine", None)
+        if engine is not None:
+            try:
+                await engine.dispose()
+            except Exception:  # noqa: BLE001 — teardown best-effort
+                pass