Implement max_models_by_field and unique field constraints

[Copilot]

Adds two new data-integrity features: per-field creation limits per user (max_models_by_field) and database-level unique column constraints (unique: true).

max_models_by_field

reaction:
  auth:
    require_login: true
    max_models_by_field:
      post_id: 1  # one reaction per user per post

• src/mspec/core.py: Defaults max_models_by_field to {} in all auth blocks; validates that it cannot be set without require_login: true
• src/mapp/module/model/db.py — db_model_create: After the max_models_per_user check, iterates max_models_by_field and raises MappUserError('MAX_MODELS_BY_FIELD_EXCEEDED', ...) (→ HTTP 400) when exceeded

unique field constraint

username:
  type: str
  unique: true

• db_model_create_table: Appends UNIQUE to the SQLite column definition for non-list, non-foreign-key fields with unique: true
• db_model_create: Catches sqlite3.IntegrityError and raises MappUserError('UNIQUE_CONSTRAINT_VIOLATED', 'A record with that value already exists.') (→ HTTP 400)

Tests & docs

• Added limit_d (exercises max_models_by_field) and unique_test (exercises unique: true) models to model-type-testing.yaml
• run_server_crud_for_model in test.py now asserts HTTP 400 + correct error codes for both constraint types
• docs/LINGO_MAPP_SPEC.md documents the new auth.max_models_by_field dict and unique field attribute