[ENV-LADDER] Add per-project ordered environment ladder column + shim

Replace the single default_branch with an ordered environments ladder
(list[{name,branch}]; first=head=PR target, last=prod=release target,
middle=intermediates). Migration 073 adds nullable JSONB projects.environments;
a read-time shim (roboco/models/env_branches.py) synthesizes a degenerate
single-branch ladder from default_branch when environments is null, so
behavior is unchanged until the operator declares a real split in the panel.
Wired through Project/ProjectCreate/ProjectUpdate models (normalize_environments
validator: rejects empty name/branch + dup branches), ProjectTable column,
API request/response schemas, create+update routes + service. EnvSyncEngine
+ consumer rewires follow.

alembic/versions/073_project_environments.py

@@ -0,0 +1,42 @@
+"""Per-project ordered environment ladder column.
+
+Replaces the single ``default_branch`` as the source of truth for a project's
+PR target (head rung) and release target (prod rung). The ladder is an ordered
+``list[{name, branch}]``: index 0 = head (where dev/cell/leaf PRs land), index
+-1 = prod (where the gated release executor commits + tags), middle rungs =
+intermediates (qa/stag). The ``EnvSyncEngine`` cascades prod→…→head so dev
+never falls behind prod, and the CEO-gated release promotes the full chain
+head→…→prod before bumping.
+
+Additive and nullable: an unset (null) ``environments`` falls back to a
+degenerate single-branch ladder synthesized from ``default_branch`` at read
+time (``roboco/services/env_branches.py``), so every existing project keeps
+behaving byte-for-byte as before until the operator declares a real ladder
+in the panel. ``default_branch`` is retained as the legacy/shim source.
+
+Revision ID: 073_project_environments
+Revises: 072_project_sandbox_extensions
+Create Date: 2026-07-15
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects import postgresql
+
+revision = "073_project_environments"
+down_revision = "072_project_sandbox_extensions"
+branch_labels: dict[str, str] | None = None
+depends_on: dict[str, str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "projects",
+        sa.Column("environments", postgresql.JSONB(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("projects", "environments")