refactor: tech debt reduction - extract hooks, deduplicate helpers, update deps, add CI

- Extract useCustomStepFlow hook from TreeNavigationPage (1040 → 759 lines)
- Create core/filters.py with shared tree/step visibility filters
- Create services/export_service.py from session export logic
- Add GitHub Actions CI/CD pipeline (pytest + lint + build)
- Add GIN index migration for full-text search on trees
- Update FastAPI 0.128.5, Pydantic 2.12.5, SQLAlchemy 2.0.46, +5 more
- Fix regex → pattern deprecation in Query() params

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

backend/alembic/versions/027_add_trees_fts_index.py

@@ -0,0 +1,32 @@
+"""add full-text search GIN index on trees
+
+Revision ID: 027
+Revises: 026
+Create Date: 2026-02-08
+
+Adds a GIN index for full-text search on the trees table,
+indexing the name and description columns using English text search.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '027'
+down_revision: Union[str, None] = '026'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(
+        "CREATE INDEX idx_trees_fts ON trees USING GIN ("
+        "to_tsvector('english', COALESCE(name, '') || ' ' || COALESCE(description, ''))"
+        ")"
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS idx_trees_fts")