docs: document database schema with table descriptions and ER diagram

[charannyk06]

Priority: P3 — Low

Problem

The SQLite schema is defined inline in src/server/store.ts and the Supabase schema is split across multiple migration files. There is no unified reference document describing:

• What tables exist
• What each column stores
• How tables relate to each other
• Why specific design decisions were made

Solution

Create docs/database-schema.md:

SQLite (Local — ~/.chainreview/chainreview.db)

Document each table:

### `runs`
Stores each review run.

| Column | Type | Description |
|--------|------|-------------|
| id | TEXT | UUID, primary key |
| repo_root | TEXT | Absolute path to repository root |
| status | TEXT | queued \| running \| complete \| failed |
| started_at | INTEGER | Unix timestamp |
| completed_at | INTEGER | Unix timestamp, null if not done |
| ...

### `findings`
Individual findings from agent analysis.

### `chat_messages`
Chat Q&A messages per run.

### `patch_applications`
Record of applied patches.

Supabase (Cloud — PostgreSQL)

Document tables from supabase/migrations/:

• profiles — user accounts
• api_keys — per-user API keys
• usage — request/token tracking
• subscriptions — Stripe subscription records

Mermaid ER Diagram

erDiagram
    runs ||--o{ findings : "has"
    runs ||--o{ chat_messages : "has"
    findings ||--o{ patch_applications : "has"

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Acceptance Criteria

• docs/database-schema.md documents all SQLite tables
• Supabase tables documented
• ER diagram (Mermaid)
• Linked from DEVELOPMENT.md