fix(projections): overhaul the event projection system (#10560)

This PR overhauls our event projection system to make it more robust and
prevent skipped events under high load. The core change replaces our
custom, transaction-based locking with standard PostgreSQL advisory
locks. We also introduce a worker pool to manage concurrency and prevent
database connection exhaustion.

### Key Changes

* **Advisory Locks for Projections:** Replaces exclusive row locks and
inspection of `pg_stat_activity` with PostgreSQL advisory locks for
managing projection state. This is a more reliable and standard approach
to distributed locking.
* **Simplified Await Logic:** Removes the complex logic for awaiting
open transactions, simplifying it to a more straightforward time-based
filtering of events.
* **Projection Worker Pool:** Implements a worker pool to limit
concurrent projection triggers, preventing connection exhaustion and
improving stability under load. A new `MaxParallelTriggers`
configuration option is introduced.

### Problem Solved

Under high throughput, a race condition could cause projections to miss
events from the eventstore. This led to inconsistent data in projection
tables (e.g., a user grant might be missing). This PR fixes the
underlying locking and concurrency issues to ensure all events are
processed reliably.

### How it Works

1. **Event Writing:** When writing events, a *shared* advisory lock is
taken. This signals that a write is in progress.
2.  **Event Handling (Projections):**
* A projection worker attempts to acquire an *exclusive* advisory lock
for that specific projection. If the lock is already held, it means
another worker is on the job, so the current one backs off.
* Once the lock is acquired, the worker briefly acquires and releases
the same *shared* lock used by event writers. This acts as a barrier,
ensuring it waits for any in-flight writes to complete.
* Finally, it processes all events that occurred before its transaction
began.

### Additional Information

* ZITADEL no longer modifies the `application_name` PostgreSQL variable
during event writes.
*   The lock on the `current_states` table is now `FOR NO KEY UPDATE`.
*   Fixes https://github.com/zitadel/zitadel/issues/8509

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Tim Möhlmann <tim+github@zitadel.com>

internal/query/current_state.go

@@ -130,7 +130,7 @@ func (q *Queries) checkAndLock(tx *sql.Tx, projectionName string) (name string,
 		From(currentStateTable.identifier()).
 		Where(sq.Eq{
 			CurrentStateColProjectionName.identifier(): projectionName,
-		}).Suffix("FOR UPDATE").
+		}).Suffix("FOR NO KEY UPDATE").
 		PlaceholderFormat(sq.Dollar).
 		ToSql()
 	if err != nil {