fix(forms): emit content event for public submissions

[DavidBabinec]

Fixes #372

What changed

• Emit content.entry.created after a successful public form row is persisted.
• Use the existing centralized content-event helper and { kind: 'system' } actor.
• Extend the real challenged-submission test to verify the table slug, row ID, and actor payload.
• Document public submissions in the forms, content storage, and plugin event contracts.

Why

The public form handler returned immediately after creating its row, bypassing the event channel used by every other content creation path. Notification and automation plugins could not observe successful submissions. Rejected-submission observability remains separate from this fix.

Verification

• Before: targeted test failed, 12 pass / 1 fail
• Fixed: targeted test passed, 13 pass / 0 fail
• Control without handler fix: failed, 12 pass / 1 fail
• bun run build and bun run lint (exit 0)
• bun test (exit 0, 6,725 pass / 0 fail)