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>

CLAUDE.md

@@ -40,7 +40,8 @@ app.py (asyncio loop, signals, Prometheus server)
   -> Ingestor (ingestor.py) — lifecycle: prepare_ingestor() -> loop() -> shutdown()
        -> IngestorManager (managers/ingestor_manager.py) — central coordinator
             -> ResourceManager (managers/resource_manager.py) — Redis slot leases, heartbeats, active-ingestor registry
-            -> OpcManager (managers/opc_manager.py), one instance per OPC server — connection, subscriptions, datachange callbacks
+            -> ServerReader (managers/server_reader.py), one per OPC server — SubscriptionEngine over the shared OpcConnectionPool
+            -> OpcConnectionPool (connection/pool.py) + CertProvider (connection/certs.py) — pooled connections, cert from MinIO
             -> DataManager (managers/data_manager.py) — MongoDB persistence + optional Kafka publish
 ```
 
@@ -56,7 +57,7 @@ app.py (asyncio loop, signals, Prometheus server)
 
 Any change to slot/lease counts should be traced through `manage_leases`/`manage_no_slots` — that's where the rebalancing math lives, not in `ResourceManager` itself (which just does Redis primitives).
 
-**OpcManager** owns one OPC UA `Client` connection (asyncua), handles security policy setup (cert/key-based, `SecurityPolicyBasic256`), subscriptions per tag group, and `datachange_notification` callbacks that push into `data_queue` for `DataManager` to consume. Tracks `non_receive_count` to detect silently-dead subscriptions (`check_cycles`).
+**ServerReader** (SIENTIAPDE-1974, replaced the legacy `OpcManager`) reads one OPC server through a `SubscriptionEngine`: the connection comes from the shared `OpcConnectionPool` (also used by NATS writes and gRPC), which fetches the platform certificate from MinIO via `CertProvider` and uses `SecurityPolicyBasic256Sha256` — no certificate files in the repo/image. Slots are parsed by `io/slots.py` (`parse_slot`), which accepts slot v2 (`names[]`) and converts the legacy v1 (`topics`). `datachange_notification` fans each value out to every `names` entry via `DataSink`. Subscribed connections are pinned in the pool so the idle sweep never drops them. Tracks `non_receive_count` to detect silently-dead subscriptions (`check_cycles` / `check_opc_listenning`); a lost server is shut down (pool connection evicted) and re-initialized on the next loop.
 
 **Metrics** (`ingestor/metrics.py`): all Prometheus metrics defined here, always labeled by `pod_id`. Emitted via `SientiaMonitoring.emit_metric(...)` (from `sientia_do`), not the prometheus client directly — new metrics should follow that pattern for consistency with logging/notifications.