refactor: move rate limit counters to accounts table, remove send_log

[genro]

Problem

The current send_log table creates one record per email sent, used only for rate limiting. This causes:

• Unbounded growth: 1M emails/day = 1M records/day
• No automatic purge: records accumulate indefinitely
• Inefficient queries: counting records instead of reading a value

Proposed Solution

In-memory rate limiter - no database at all for rate limiting.

class RateLimiter:
    """Counters in memory, zero DB."""
    
    def __init__(self):
        # {account_id: {minute_ts: count, hour_ts: count, day_ts: count}}
        self._counters: dict[str, dict[str, int]] = {}
    
    def check_and_increment(self, account_id: str, limits: dict) -> bool:
        """Check limits and increment. Returns True if allowed."""
        now = int(time.time())
        # ... rolling counters logic

Benefits

Aspect	DB	In-Memory
Latency	~1ms query	~1μs dict lookup
I/O	Write every email	Zero I/O
Complexity	Schema, migrations	One dict
Crash recovery	Keeps counters	Reset to zero

Crash Recovery

If service restarts, counters reset to zero. This is:

• Conservative: can send again after restart
• Self-healing: limits are per minute/hour/day, realigns quickly
• Same as Redis: with TTL keys

Tasks

• Create RateLimiter class with in-memory counters
• Modify rate limit check to use RateLimiter instead of send_log
• Remove send_log table and entities/send_log/ directory
• Update tests