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>

ingestor/connection/pool.py

@@ -26,6 +26,10 @@ class OpcConnectionPool(SientiaMonitoring):
         self.cert_provider = cert_provider
         self.clock = MonotonicClock()
         self._connections: dict[str, tuple[Client, float]] = {}
+        # Active subscriptions per server. A subscribed connection only receives
+        # callbacks and never calls get(), so without a pin release_idle() would
+        # evict it as idle and silently stop the reads (SIENTIAPDE-1974 G4).
+        self._pins: dict[str, int] = {}
         # ponytail: one Lock per server_id, never cleaned up — server set is
         # small and bounded. Add eviction of stale locks only if it grows.
         self._locks: dict[str, asyncio.Lock] = {}
@@ -96,15 +100,36 @@ class OpcConnectionPool(SientiaMonitoring):
             tags={'pod_id': self.pod_id, 'server_name': server_id, 'server_url': url},
         )
 
+    def pin(self, server_id: str) -> None:
+        """Mark one more active subscription on `server_id` (never evicted as idle)."""
+        self._pins[server_id] = self._pins.get(server_id, 0) + 1
+
+    def unpin(self, server_id: str) -> None:
+        """Drop one active subscription mark from `server_id`."""
+        count = self._pins.get(server_id, 0) - 1
+        if count > 0:
+            self._pins[server_id] = count
+        else:
+            self._pins.pop(server_id, None)
+
     async def release_idle(self, max_idle_s: float):
-        """Disconnect and drop clients unused for longer than `max_idle_s`."""
+        """Disconnect and drop clients unused for longer than `max_idle_s`,
+        except the ones pinned by an active subscription."""
         now = self.clock.elapsed()
         idle_ids = [
-            sid for sid, (_, last_used) in self._connections.items() if now - last_used > max_idle_s
+            sid
+            for sid, (_, last_used) in self._connections.items()
+            if now - last_used > max_idle_s and not self._pins.get(sid)
         ]
         for server_id in idle_ids:
             await self._evict(server_id)
 
+    async def evict(self, server_id: str):
+        """Disconnect `server_id` now (lost server or changed config); the next
+        get() reconnects. No-op when the server has no pooled client."""
+        if server_id in self._connections:
+            await self._evict(server_id)
+
     async def close_all(self):
         """Disconnect every pooled client (e.g. on shutdown)."""
         # list() snapshots the keys: _evict() pops from self._connections,
@@ -122,7 +147,7 @@ class OpcConnectionPool(SientiaMonitoring):
 
     async def _disconnect_with_retries(self, client: Client, server_id: str) -> None:
         """Tries 5 times to disconnect, with a delay of 100ms x try (mirrors
-        OpcManager.disconnection_fallback)."""
+        the legacy OpcManager.disconnection_fallback)."""
         for i in range(5):
             try:
                 await client.disconnect()