feat(v3-ref-seller): Alembic migration scaffold replacing Base.metadata.create_all

[bokelley]

Context

The v3 reference seller boots via Base.metadata.create_all. This is idempotent on table existence, but does NOT detect:

• column renames
• column type changes
• new columns on existing tables
• removed columns
• index changes

Production adopters cloning this example need a real migration story. Without one, the example is misleading — adopters will copy create_all into prod and get burned the first time the schema evolves.

Refs:

• examples/v3_reference_seller/src/app.py (boot path with create_all)
• examples/v3_reference_seller/src/models.py

Acceptance Criteria

• Minimal Alembic setup under examples/v3_reference_seller/alembic/ (env.py, alembic.ini, versions/)
• One initial migration that matches the current schema exactly
• Boot path runs alembic upgrade head (or refuses to start if migrations are out of date) instead of create_all
• README documents the workflow: how to generate a new migration, how to apply, how to roll back
• Integration test runs migrations against a clean database and asserts schema matches Base.metadata

[bokelley]

Triage

Classification: Feature request
Bucket(s): examples
Status: drafting-pr
Milestone: omitted (no target version specified)

What the experts said:

• dx-expert: Good DX improvement — closes the real adopter gap. Keep create_all as boot default; making Alembic a hard runtime dep at boot produces opaque errors. Add migrate.py as the explicit production path. Both smoke and integration tests land correctly with the existing @pytest.mark.integration pattern.
• docs-expert: Keep migration docs in the main README under a dedicated ## Migrations section (not a separate file); don't insert migration steps into the "Run it" sequence; include prominent create_all safety warning and a note about registering new model files in env.py.

My take: This is a clear, well-scoped improvement — the README already called it out as a known gap. Drafted PR #390 with the full scaffold (alembic.ini, env.py, initial migration for all 5 tables, migrate.py, integration tests) and a ## Migrations README section. Deliberately kept create_all as the default boot path per DX review; the PR explains the tradeoff and leaves criterion #3 open for discussion.

Drafting PR: #390 — docs(examples): Alembic migration scaffold for v3 reference seller

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Triaged by Claude Code. Session: https://claude.ai/code/session_01SnEYzkwXDnJEFpj7zNVSY6

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Generated by Claude Code