Parameterise PostgreSQL schema throughout the backend

[will-langdale]

MB__SERVER__POSTGRES__DB_SCHEMA was only respected by the SQLAlchemy ORM metadata. All Alembic migrations, several raw SQL strings, and the ingest utilities were hardcoded to "mb".

🛠️ Changes proposed in this pull request

• env.py: pass db_schema into Alembic config so migrations can read it
• All 25 migration files: replace schema="mb" kwargs, inline FK strings (e.g. ["mb.table.col"]), and raw SQL table references with the parameterised value via op.get_context().config.get_main_option()
• Adds an Alembic migration hook to ensure this happens in future migrations
• First migration: remove CREATE/DROP SCHEMA — the operator is now responsible for creating the schema before running migrations
• db.py: parameterise the pre-alembic recovery DROP SCHEMA
• orm.py: fix hardcoded schema on the upload_stages Enum type
• insert.py, adapter/eval.py: fix schema_name="mb" on temp table calls
• justfile: use $MB__SERVER__POSTGRES__DB_SCHEMA in the drop recipe

👀 Guidance to review

Alembic post migration generation hook in action:

(matchbox-db) willlangdale@DBT001350 ~/D/matchbox (claude/awesome-mccarthy-p5Eb8)> just migrate generate "Add level_2."
uv run alembic --config "src/matchbox/server/postgresql/alembic.ini" revision --autogenerate -m "Add level_2."
...
INFO  [alembic.autogenerate.compare.tables] Detected added column 'mb.step_from.level_2'
  Generating /Users/willlangdale/Dev/matchbox/src/matchbox/server/postgresql/alembic/versions/5e33e5844258_add_level_2.py ...  done
  Running post write hook 'fix_schema' ...
  done
  Running post write hook 'ruff_format' ...
1 file reformatted
  done
  Running post write hook 'ruff_lint' ...
Found 2 errors (2 fixed, 0 remaining).
  done

"""Add level_2.

Revision ID: 5e33e5844258
Revises: 65dd8d1fe119
Create Date: 2026-06-09 13:11:20.645400

"""

from collections.abc import Sequence

import sqlalchemy as sa
from alembic import op

# revision identifiers, used by Alembic.
revision: str = "5e33e5844258"
down_revision: str | None = "65dd8d1fe119"
branch_labels: str | Sequence[str] | None = None
depends_on: str | Sequence[str] | None = None


def upgrade() -> None:
    """Upgrade schema."""
    schema = op.get_context().config.get_main_option("db_schema")
    # ### commands auto generated by Alembic - please adjust! ###
    op.add_column(
        "step_from", sa.Column("level_2", sa.INTEGER(), nullable=False), schema=schema
    )
    # ### end Alembic commands ###


def downgrade() -> None:
    """Downgrade schema."""
    schema = op.get_context().config.get_main_option("db_schema")
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_column("step_from", "level_2", schema=schema)
    # ### end Alembic commands ###

🤖 AI declaration

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