feat: add procedural flows with intake forms, navigation, and seed templates

Adds a new "procedural" tree type for linear step-by-step project workflows
(domain controller setup, M365 onboarding, VPN config, etc). Includes intake
form builder, two-panel step navigation, variable resolution, procedural
exports, 3 seed templates, and UI rename from "Trees" to "Flows".

Also archives 19 implemented plan docs and creates deferred features backlog.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/tree.py

@@ -28,6 +28,10 @@ class Tree(Base):
             "status IN ('draft', 'published')",
             name='ck_trees_status'
         ),
+        CheckConstraint(
+            "tree_type IN ('troubleshooting', 'procedural')",
+            name='ck_trees_tree_type'
+        ),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(
@@ -47,7 +51,20 @@ class Tree(Base):
         nullable=True,
         index=True
     )
+    tree_type: Mapped[str] = mapped_column(
+        String(20),
+        nullable=False,
+        default='troubleshooting',
+        server_default='troubleshooting',
+        index=True,
+        comment="Tree type: troubleshooting (branching decision tree) or procedural (linear step-by-step flow)"
+    )
     tree_structure: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False)
+    intake_form: Mapped[Optional[list[dict[str, Any]]]] = mapped_column(
+        JSONB,
+        nullable=True,
+        comment="Intake form field definitions for procedural flows (JSONB array)"
+    )
     author_id: Mapped[Optional[uuid.UUID]] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("users.id"),