Generate embed_key at project creation time

[Copilot]

Addresses feedback from PR #116: newly published projects lacked embed_key until the owner viewed them, preventing immediate VIP link sharing.

Changes

• publish_handler: Generate embed_key during INSERT using encode(gen_random_bytes(24), 'base64')
• get_project handler: Retain lazy generation as fallback for pre-existing projects

// Before: embed_key was NULL for new projects
INSERT INTO projects (slug, image_tag, markdown, owner_id, owner_username, shell, embed_token) 
VALUES ($1, $2, $3, $4, $5, $6, gen_random_uuid()::text)

// After: embed_key generated immediately
INSERT INTO projects (slug, image_tag, markdown, owner_id, owner_username, shell, embed_token, embed_key) 
VALUES ($1, $2, $3, $4, $5, $6, gen_random_uuid()::text, encode(gen_random_bytes(24), 'base64'))

VIP links now work immediately after publishing without requiring the owner to view the project first.

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