Support multiple reporting organizations (multi-tenant NCMEC)

[nullEFFORT]

Currently single-tenant: one set of NCMEC credentials from env vars, hardcoded reporter identity. No org model.

For multi-org support, need:

• Organization model with ESP ID, org name, NCMEC credentials (encrypted at rest)
• Operator to Organization FK
• Session to Organization context for audit trail
• Per-org reporter identity in XML report builder
• Per-org NCMECClient instantiation with org-specific credentials
• Credential rotation support
• Optional: per-org database isolation