[ENHANCEMENT] Standardize Error Handling in Event Producer to Enable Retries and Filter Poison Pills

**Description:**
Currently, the `ProcessSearch` function in the Event Producer (`workers/event_producer/pkg/producer/producer.go`) only wraps errors from `AcquireLock` with `event.ErrTransientFailure`. All other errors (Database reads/writes, GCS operations, external API calls) are returned as standard errors.

**The Problem:**
The Pub/Sub subscriber adapter is configured to **ACK** (drop) any message where the handler returns a non-transient error. This means that a temporary network glitch (e.g., Spanner read timeout, GCS 503) will cause the notification job to be **permanently lost** instead of retried.

https://github.com/GoogleChrome/webstatus.dev/blob/04a94439ffd32f583caa05143a215a0cf748ffac/lib/gcppubsub/client.go#L68-L82

Conversely, simply wrapping *all* errors as transient would cause "poison pills" (e.g., invalid Saved Search IDs) to loop infinitely.

**Objective:**
We need to implement a robust error handling strategy that retries by default (to catch network issues) but fails fast on known invalid states.

**Proposed Changes:**

1. **Define Standard Errors:** Create `lib/workertypes/errors.go` to define sentinel errors for adapters:
* `var ErrEntityNotFound = errors.New("entity not found")`
* `var ErrInvalidData = errors.New("invalid data format")`

2. **Update Adapters:** Modify Spanner and GCS adapters to return these specific errors where appropriate (e.g., `GetLatestEvent` returning `ErrEntityNotFound` if the search ID is invalid).
3. **Update ProcessSearch Logic:** Refactor error handling in `ProcessSearch` to follow this pattern:
* **Permanent Failure Check:** If the error matches `workertypes.ErrEntityNotFound` or `workertypes.ErrInvalidData`, return it directly (triggering an ACK/Drop).
* **Transient Default:** For all other errors (unknown or network-related), wrap them with `event.ErrTransientFailure` (triggering a NACK/Retry).

**Acceptance Criteria:**

* Transient failures (network blips) in DB, Storage, or Differ now trigger a retry (NACK).
* Permanent failures (missing search ID, bad data) trigger an immediate failure (ACK) and are not retried.