chore: archive old plan docs + add survey foundation files

Move completed plan docs to docs/plans/archive/. Add survey migration 046
and reference HTML/plan files.

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

backend/alembic/versions/046_add_survey_responses_table.py

@@ -0,0 +1,32 @@
+"""Add survey_responses table.
+
+Revision ID: 046
+Revises: 045
+Create Date: 2026-03-04
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+revision: str = "046"
+down_revision: str = "045"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "survey_responses",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True),
+        sa.Column("respondent_name", sa.String(255), nullable=True),
+        sa.Column("responses", JSONB, nullable=False),
+        sa.Column("ip_address", sa.String(45), nullable=True),
+        sa.Column("user_agent", sa.Text, nullable=True),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("survey_responses")