fix(file-service): restore share notifications

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Summary

Symptom / blast radius: file_shared notification publishes were routed to a nonexistent SNS topic ending in -v2, so shares returned an SNS error and recipients did not receive notifications.

Root cause (config/env): the file-service image baked FILE_SHARE_EVENT_ALWAYS_FAIL=true; EventPublisher::file_shared therefore selected SNS_TOPIC_ARN-v2 instead of the configured SNS topic.

Fix: remove the enabled image default so deployed file-service instances use the existing off-by-default application configuration and publish shares to SNS_TOPIC_ARN. Add a regression guard covering modern and legacy ENV syntax, truthy/quoted values, multi-assignment declarations, and backslash continuations. Upload failure and seeded-content settings are unchanged.

Verification:

• cargo test: 26 passed
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• Mutation check: a backslash-continued truthy Dockerfile assignment makes the regression guard fail
• Built image contains no FILE_SHARE_EVENT_ALWAYS_FAIL
• API and browser shares of Annual Planning 2027.txt return 201 and publish file_shared to otterworks-events; zero -v2 or publish-failure log entries
• Recipient sees the shared file and in-app notification
• Recording: https://app.devin.ai/attachments/77335a43-cb4a-4c93-be4a-bc80c16fa257/pr253-share-edited.mp4

Follow-up observed: after the successful share, the owner's "People with access" list did not refresh, although recipient access and notification delivery were confirmed. This appears separate from the SNS outage and is not changed here.

Related independent work, left unchanged: #251, #248, #245.

Link to Devin session: https://app.devin.ai/sessions/b9ea2445f63348559338e25689db482f

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