docs: Add ADR-020 and migration system implementation guidance

Architecture documentation for automatic database migrations.

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Co-Authored-By: Claude <noreply@anthropic.com>

docs/reports/2025-11-19-migration-implementation-quick-reference.md

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+# Migration System - Quick Reference Card
+
+**TL;DR**: Add fresh database detection to `migrations.py` to solve chicken-and-egg problem.
+
+## The Problem
+
+- `SCHEMA_SQL` includes `code_verifier` column (line 60, database.py)
+- Migration 001 tries to add same column
+- Fresh databases fail: "column already exists"
+
+## The Solution
+
+**SCHEMA_SQL = Target State** (complete current schema)
+- Fresh installs: Execute SCHEMA_SQL, skip migrations (already at target)
+- Existing installs: Run migrations to reach target
+
+## Code Changes Required
+
+### 1. Add to `migrations.py` (before `run_migrations`):
+
+```python
+def is_schema_current(conn):
+    """Check if database schema matches current SCHEMA_SQL"""
+    try:
+        cursor = conn.execute("PRAGMA table_info(auth_state)")
+        columns = [row[1] for row in cursor.fetchall()]
+        return 'code_verifier' in columns
+    except sqlite3.OperationalError:
+        return False
+```
+
+### 2. Modify `run_migrations()` in `migrations.py`:
+
+After `create_migrations_table(conn)`, before applying migrations, add:
+
+```python
+# Check if this is a fresh database
+cursor = conn.execute("SELECT COUNT(*) FROM schema_migrations")
+migration_count = cursor.fetchone()[0]
+
+# Discover migration files
+migration_files = discover_migration_files(migrations_dir)
+
+# Fresh database detection
+if migration_count == 0 and is_schema_current(conn):
+    # Mark all migrations as applied (schema already current)
+    for migration_name, _ in migration_files:
+        conn.execute(
+            "INSERT INTO schema_migrations (migration_name) VALUES (?)",
+            (migration_name,)
+        )
+    conn.commit()
+    logger.info(f"Fresh database: marked {len(migration_files)} migrations as applied")
+    return
+```
+
+### 3. Optional Helpers (add to `migrations.py` for future use):
+
+```python
+def table_exists(conn, table_name):
+    cursor = conn.execute(
+        "SELECT name FROM sqlite_master WHERE type='table' AND name=?",
+        (table_name,)
+    )
+    return cursor.fetchone() is not None
+
+def column_exists(conn, table_name, column_name):
+    try:
+        cursor = conn.execute(f"PRAGMA table_info({table_name})")
+        columns = [row[1] for row in cursor.fetchall()]
+        return column_name in columns
+    except sqlite3.OperationalError:
+        return False
+```
+
+## Test It
+
+```bash
+# Test 1: Fresh database
+rm data/starpunk.db && uv run flask --app app.py run
+# Expected: "Fresh database: marked 1 migrations as applied"
+
+# Test 2: Legacy database (before PKCE)
+# Create old schema, run app
+# Expected: "Applied migration: 001_add_code_verifier..."
+```
+
+## All Other Questions Answered
+
+- **Q2**: schema_migrations only in migrations.py ✓ (already correct)
+- **Q3**: Accept non-idempotent SQL, rely on tracking ✓ (already works)
+- **Q4**: Flexible filename validation ✓ (already implemented)
+- **Q5**: Automatic transition via Q1 solution ✓
+- **Q6**: Helpers provided for advanced use ✓ (see above)
+- **Q7**: SCHEMA_SQL is target state ✓ (no changes needed)
+
+## Full Details
+
+See: `/home/phil/Projects/starpunk/docs/reports/2025-11-19-migration-system-implementation-guidance.md`
+
+## Architecture Reference
+
+See: `/home/phil/Projects/starpunk/docs/decisions/ADR-020-automatic-database-migrations.md`
+(New section: "Developer Questions & Architectural Responses")