SIENTIAPDE-1974: feat(read): slot v2 reads via SubscriptionEngine, OPC cert from MinIO

Swaps the legacy OpcManager read path for the gateway's SubscriptionEngine over
the shared OpcConnectionPool. The OPC client certificate now always comes from
MinIO + Mongo (CertProvider, Basic256Sha256) — no more certs/ shipped in the
Gitea repository, so a new version branch no longer breaks the reads.

- io/slots.py: parse_slot accepts slot v2 (names[]) and converts the legacy
  v1 (tag_name + topics; cert_path => use_security, path dropped), so gateway
  and Orchestrator can be deployed in any order.
- managers/server_reader.py: new per-server reader with the OpcManager
  interface the IngestorManager already drives (connect, create_subscription,
  subscribe, unsubscribe, check_cycles, check_opc_listenning, shutdown).
- IngestorManager owns the read path: lazy-MinIO CertProvider, the pool and a
  read DataSink, built without touching the network. Slots from Redis are
  normalized on lease and refresh; a malformed slot is notified and the last
  good config is kept.
- GatewayRuntime borrows the pool (writes/gRPC) instead of building it, and no
  longer closes it on stop: a Postgres/NATS failure can't stop the reads.
- pool: pin/unpin per active subscription so release_idle never evicts a
  subscribed connection; public evict() for lost servers / changed config.
- SubscriptionEngine pins on subscribe, unpins on unsubscribe, and drops a
  subscription locally even when the server-side delete fails.
- DataSink: model_id only written when the name has one, keeping v1 documents
  identical to the legacy ones.
- config: CERT_BUCKET default is the legacy streamlit-connectors bucket (D7).
- Removes OpcManager, its tests and scripts/generate-and-push-opc-cert.sh;
  README/CLAUDE.md document the MinIO certificate and the slot v2 format.

Tests: 218 unit tests pass; ruff check/format clean. Local end-to-end against
an asyncua server: v1 slot with two topics -> one doc per collection, legacy
doc shape, idle sweep keeps the subscription, lost-server shutdown and
re-initialization work.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/connection/test_pool.py

@@ -182,3 +182,69 @@ async def test_concurrent_get_same_server_connects_once():
     assert c1 is c2 is clients[0]
     assert ctor.call_count == 1
     assert clients[0].connect.await_count == 1
+
+
+@pytest.mark.asyncio
+async def test_release_idle_keeps_pinned_connection():
+    """A subscribed connection only gets callbacks (never get()), so it looks idle:
+    the pin keeps release_idle from dropping it (SIENTIAPDE-1974 G4)."""
+    pool = make_pool()
+    fake_client = AsyncMock()
+    with patch('ingestor.connection.pool.Client', return_value=fake_client):
+        await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+
+    pool.pin('srv')
+    await pool.release_idle(max_idle_s=0)
+
+    assert 'srv' in pool._connections
+    fake_client.disconnect.assert_not_awaited()
+
+
+@pytest.mark.asyncio
+async def test_release_idle_evicts_after_last_unpin():
+    pool = make_pool()
+    fake_client = AsyncMock()
+    with patch('ingestor.connection.pool.Client', return_value=fake_client):
+        await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+
+    pool.pin('srv')
+    pool.pin('srv')  # two slots subscribed on the same server
+    pool.unpin('srv')
+    await pool.release_idle(max_idle_s=0)
+    assert 'srv' in pool._connections  # one subscription still active
+
+    pool.unpin('srv')
+    await pool.release_idle(max_idle_s=0)
+    assert 'srv' not in pool._connections
+    fake_client.disconnect.assert_awaited_once()
+
+
+def test_unpin_never_goes_negative():
+    pool = make_pool()
+
+    pool.unpin('srv')
+    pool.pin('srv')
+
+    assert pool._pins == {'srv': 1}
+
+
+@pytest.mark.asyncio
+async def test_evict_disconnects_now_and_next_get_reconnects():
+    pool = make_pool()
+    first, second = AsyncMock(), AsyncMock()
+    with patch('ingestor.connection.pool.Client', side_effect=[first, second]):
+        await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+        await pool.evict('srv')
+        client = await pool.get('srv', {'url': 'opc.tcp://h:4840', 'use_security': False})
+
+    first.disconnect.assert_awaited_once()
+    assert client is second
+
+
+@pytest.mark.asyncio
+async def test_evict_unknown_server_is_noop():
+    pool = make_pool()
+
+    await pool.evict('nope')
+
+    pool.emit_metric.assert_not_awaited()