feat(server): add tenant scaffolding and targets schema (1/7)

[abhinav-galileo]

Summary

• Adds additive tenant_id scaffolding (String(64), default "default-tenant") to agents, controls, policies, agent_controls, agent_policies, with DB-level server_default so unscoped writes keep working.
• Adds new targets and target_controls tables for attaching controls to typed, tenant-scoped targets (e.g. a log stream).
• Backfills existing rows to the default tenant and flips columns to NOT NULL.
• Behavior-preserving: read paths remain unscoped; new target tables exist but are not yet wired into runtime resolution or management APIs.

Intentional deviations and decisions

• target_type instead of bare type for clarity and to avoid shadowing Python's builtin.
• policy_controls.tenant_id omitted: tenant scope is inherited transitively via policy_id / control_id.
• tenant_id indexes omitted: read paths are unscoped in this phase, so an index would carry write cost for no benefit. Add alongside the PR that introduces tenant-scoped reads.
• FK cascades on target_controls: target_id uses ON DELETE CASCADE (attachment has no meaning without the target); control_id uses the default restrictive behavior so control deletion does not silently fan into attachments.

Deferrals

• updated_at on new tables: omitted - no live repo pattern for automatic maintenance. Defer until a pattern is chosen repo-wide.
• control_execution_events.tenant_id: deferred; observability tables are out of scope here.
• tenants table, target_targets DAG, tenant-scoped uniqueness on agent/control/policy names, lazy target upsert, runtime resolution, SDK/UI changes: all deferred to follow-up PRs.

Test plan

• make check clean locally (lint + mypy + tests across all packages)
• New migration tests cover: backfill correctness across all 5 tables, new table creation with uniqueness, post-migration server_default behavior, downgrade completeness
• New ORM tests cover: Target / TargetControl round-trip, (tenant_id, target_type, external_id) uniqueness, (target_id, control_id) uniqueness, ON DELETE CASCADE behavior, implicit default tenant on unscoped writes for Agent / Control / Policy

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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