Scaffold FastAPI + MySQL + Keycloak service with devcontainer

Sets up the project skeleton:

- FastAPI app factory with lifespan, request-id middleware, and RFC 9457
  problem+json error handlers
- Async SQLAlchemy 2.0 over MySQL (asyncmy), with a constraint naming
  convention in place before the first migration and async Alembic
- Keycloak as a pure resource server: OIDC discovery, cached JWKS with
  rotation-aware refresh, and require_roles dependencies
- Devcontainer running MySQL 8.4 and Keycloak 26.7 as compose siblings,
  with the realm (clients, roles, test users) imported on first boot
- Test suite covering the endpoints plus the token validator itself,
  exercised against a locally generated RSA keypair
- uv packaging, ruff, mypy --strict, pre-commit, Gitea CI, prod Dockerfile

Two Keycloak-in-containers traps are handled explicitly and documented in
the README: the issuer/internal-URL split (the browser sees localhost:8080,
the API sees keycloak:8080) and the audience mapper that stops Keycloak
issuing tokens with aud=account.

The devices resource is a placeholder proving the routing -> auth -> ORM ->
migration path end to end; replace it with the real domain.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/v2x_server/db/base.py

@@ -0,0 +1,43 @@
+"""Declarative base and shared column mixins."""
+
+from __future__ import annotations
+
+import datetime as dt
+
+from sqlalchemy import DateTime, MetaData, func
+from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column
+
+# Deterministic constraint names. This has to be in place *before* the first
+# migration: MySQL will otherwise invent its own names, and Alembic autogenerate
+# then produces diffs referencing constraints it cannot reliably drop.
+NAMING_CONVENTION = {
+    "ix": "ix_%(column_0_label)s",
+    "uq": "uq_%(table_name)s_%(column_0_name)s",
+    "ck": "ck_%(table_name)s_%(constraint_name)s",
+    "fk": "fk_%(table_name)s_%(column_0_name)s_%(referred_table_name)s",
+    "pk": "pk_%(table_name)s",
+}
+
+
+class Base(DeclarativeBase):
+    metadata = MetaData(naming_convention=NAMING_CONVENTION)
+
+
+class TimestampMixin:
+    """Server-side created/updated timestamps.
+
+    Defaults are rendered by MySQL rather than Python so rows written outside
+    the application (migrations, manual SQL) are stamped consistently.
+    """
+
+    created_at: Mapped[dt.datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        nullable=False,
+    )
+    updated_at: Mapped[dt.datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )