feat(common): add retriability signal to Result pattern Err type (#121)

[b3lz3but]

Summary

Adds a retriable: bool = False field to the Err dataclass so callers can distinguish transient errors (DB timeout, lock contention) from permanent ones (validation failure, business rule violation).

Closes #121

Change

# Before
@dataclass(frozen=True)
class Err[E]:
    error: E

# After
@dataclass(frozen=True)
class Err[E]:
    error: E
    retriable: bool = False

Backward compatibility

All 577 existing Err("message") call sites continue to work unchanged — the default retriable=False is applied automatically. No migration needed.

Usage

# Transient error — caller may retry
return Err("database connection timeout", retriable=True)

# Permanent error — retry will always fail (default)
return Err("invoice amount must be positive")

# Django-Q task can check:
result = refund_service.process_refund(order_id)
if result.is_err() and result.retriable:
    raise result.unwrap_err()  # Re-raise for Django-Q retry

Files changed

• apps/common/types.py — Added retriable field to Err dataclass (+5 lines)
• tests/common/test_result_types.py — 24 new tests covering Ok/Err behavior and retriable signal

Test plan

• 24 Result type tests pass
• All pre-commit hooks pass
• DCO sign-off

🤖 Generated with Claude Code

[b3lz3but]

@mostlyvirtual — small arch-debt cleanup, requesting review.