Phase 1: Migrationsaudit + Forward-Migrationen 0093-0096

Audit (docs/migration_history_audit.md):
- files.size_bytes: INTEGER (Alembic) vs BIGINT (Produktion/Plugin)
- GIN-Indizes: Fehlendes USING GIN in Alembic 0002
- guest_users: ix_guest_users_email_tenant fehlt UNIQUE in Alembic 0059
- plugins.name: Doppelter Unique-Index in Produktion

Forward-Migrationen:
- 0093: files.size_bytes INTEGER → BIGINT
- 0094: GIN-Indizes reparieren + plugins.name doppelten Index entfernen
- 0095: guest_users email+tenant_id UNIQUE INDEX (mit Dubletten-Check)
- 0096: Workspace tenant_integrity (tenant-bound FKs)

Tests: 41/41 bestanden (17 Workspace + 24 Command)
Alembic Head: 0096

alembic/versions/0095_fix_guest_users_unique.py

@@ -0,0 +1,54 @@
+"""Fix guest_users email+tenant_id unique index.
+
+Alembic 0059 created ix_guest_users_email_tenant as a normal (non-unique) index.
+The SQLAlchemy model defines it as unique=True, and production already has
+a UNIQUE INDEX. This migration aligns Alembic with production.
+
+Before creating the unique index, checks for duplicate (email, tenant_id) pairs.
+If duplicates exist, the migration aborts with a data cleanup report.
+
+Revision ID: 0095
+Revises: 0094
+"""
+
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0095"
+down_revision = "0094"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Check for duplicates before creating unique index
+    conn = op.get_bind()
+    duplicates = conn.execute(
+        sa.text(
+            "SELECT email, tenant_id, count(*) FROM guest_users "
+            "GROUP BY email, tenant_id HAVING count(*) > 1"
+        )
+    ).fetchall()
+
+    if duplicates:
+        raise RuntimeError(
+            f"Cannot create unique index: {len(duplicates)} duplicate (email, tenant_id) pairs found. "
+            "Data cleanup required before migration."
+        )
+
+    # Drop the non-unique index and recreate as unique
+    op.execute("DROP INDEX IF EXISTS ix_guest_users_email_tenant")
+    op.execute(
+        "CREATE UNIQUE INDEX IF NOT EXISTS ix_guest_users_email_tenant "
+        "ON guest_users (email, tenant_id)"
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_guest_users_email_tenant")
+    op.execute(
+        "CREATE INDEX IF NOT EXISTS ix_guest_users_email_tenant "
+        "ON guest_users (email, tenant_id)"
+    )