fix(tasks) Add rpc_secret to taskworker

[markstory]

In production we need an RPC secret to sign requests to taskbroker. Instead of using the existing shared secret between launchpad and sentry, we should use a different key.

[sentry]

Sentry Build Distribution

App Name	App ID	Version	Configuration	Install Page
Hacker News	com.emergetools.hackernews	1.0.2 (13)	Release	Install Build

[sentry]

Bug: os.getenv("TASKWORKER_SHARED_SECRET") is called without a default value or validation. If the environment variable is not set, None is passed as the rpc_secret, likely causing runtime failures.
_{Severity: HIGH}

Suggested Fix

Add a validation check to ensure the value from os.getenv("TASKWORKER_SHARED_SECRET") is not None before calling app.set_config(). If the value is missing, raise a RuntimeError to fail fast, similar to the pattern used for LAUNCHPAD_RPC_SHARED_SECRET elsewhere in the codebase.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: src/launchpad/worker/app.py#L104-L106

Potential issue: The `rpc_secret` configuration is set using
`os.getenv("TASKWORKER_SHARED_SECRET")` without a default value or subsequent
validation. If this environment variable is not set, the function returns `None`, which
is then passed to `app.set_config()`. Since the secret is described as mandatory for
production, passing `None` to the `taskbroker-client` library will likely result in a
runtime `TypeError` or silent authentication failures when the worker attempts to
communicate with the taskbroker. This contrasts with existing patterns in the codebase
where a missing secret raises a `RuntimeError`.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[markstory]

None is fine.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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