Demo mode for all ingestion connectors

[askmy-stack]

Background

Identified during OSS audit v2 of startupintel.

Problem Statement

API keys required.

Proposed Solution

DEMO_MODE fixture JSON.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

startupintel/ingestion/

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Suggested labels: priority:high, difficulty:intermediate, category:feature
Milestone: v2.0-features
OSS Audit: Master Backlog v2