docs: establish product, domain, architecture, and delivery baseline

[TechAaroorian]

Summary

Establishes the design-documentation baseline for SlotSyncro before the next implementation phase.

Included

• Product overview and feature inventory
• User journeys, use cases, and business rules
• Current and proposed domain models
• ER diagrams, constraints, and migration direction
• Modular-monolith architecture decision
• System architecture and runtime boundaries
• Vercel deployment architecture and ADR
• Vertical implementation roadmap
• Content opportunities discovered during design

Key decisions

• Use a modular monolith for the product application
• Make Meeting the shared outcome of direct booking and poll finalization
• Introduce poll-scoped candidates, participants, and preferences
• Use personal workspaces as the initial ownership boundary
• Separate notification intent and delivery attempts from meeting state
• Deploy marketing and product as separate Vercel projects
• Implement the poll-to-meeting journey as the first modular vertical slice

Validation

• Documentation links resolve
• Markdown code fences are balanced
• git diff --check passes
• Current, partial, and proposed behavior are distinguished

Notes

This PR contains documentation only. It does not change application behavior or the database schema.

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	100%	88 / 88
🔵	Statements	100%	89 / 89
🔵	Functions	100%	13 / 13
🔵	Branches	100%	33 / 33

File Coverage

No changed files found.

Generated in workflow #20 for commit e61de8b by the Vitest Coverage Report Action