Add register_encoder hook for custom API value serialization (#136)

[vsdudakov]

Closes #136. Also delivers the configurable-format enhancement noted in #135.

What

Adds a small user-facing registry so you can control how a given type is serialized in every admin API response — a custom datetime format, an Enum's label, a domain value object, etc.

import datetime
from fastadmin import register_encoder

# Render every datetime as "2024-01-31 14:05" instead of ISO 8601.
register_encoder(datetime.datetime, lambda dt: dt.strftime("%Y-%m-%d %H:%M"))

Why

Distilled from the fork analysis of this repo. The alex-averin fork added a CUSTOM_ENCODERS dict wired into FastAPI responses (commit, "Add custom encoders for fastapi"). This PR generalizes that idea into a framework-agnostic hook.

Approach

Rather than patch each of the three framework JSON encoders (FastAPI / Flask / Django), custom encoders are applied at the single framework-agnostic serialization point — BaseModelAdmin.serialize_obj_attributes — so all frameworks and ORMs benefit uniformly, and the values reaching each framework's response layer are already JSON-safe.

• fastadmin/api/encoders.py — register_encoder / unregister_encoder / clear_encoders / apply_custom_encoders over a CUSTOM_ENCODERS registry.
• fastadmin/models/base.py — apply custom encoders before the built-in Decimal formatting; a matching encoder takes precedence.
• fastadmin/__init__.py — export register_encoder / unregister_encoder from the package root.
• Matching is isinstance in registration order (register more specific types before base types).

Relationship to #135

The concrete "datetime read error" that three forks patched is already fixed in the current codebase (all three frameworks serialize datetime/date/time to ISO 8601, and base.py deserializes the date/time pickers). The one remaining ask in #135 — configurable output format — is delivered here: register_encoder(datetime.datetime, ...).

Tests

• tests/api/test_encoders.py — registry unit tests (match order, override, unregister, clear).
• tests/models/test_base.py — integration: encoder applied during serialization + overrides the built-in Decimal formatting.
• Full suite: 1975 passed. ruff check, ruff format --check, and ty check all clean.

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