Use Redis INCR for atomic counters and build a sliding rate limiter

Documented steps

1. Atomic increment: `INCR mykey` - creates the key at 0 then increments (returns the new value). Non-existent key counts as 0 first.
2. Increment by an amount: `INCRBY mykey 5`; decrement with DECR/DECRBY.
3. Counter + expiry for time windows: `INCR pageviews:user:123:2026-08-15` then `EXPIRE ... 86400` so the key cleans itself up.
4. Rate limiter (per-second per-IP) using MULTI/EXEC to make INCR+EXPIRE atomic: `MULTI` -> `INCR <ip>:<ts>` -> `EXPIRE <ip>:<ts> 10` -> `EXEC`; if the INCR result > limit, reject.
5. Safer single-counter fixed-window: `INCR <ip>`; if result == 1 set `EXPIRE <ip> 1`; reject when value > limit - but note the race unless both are in a Lua script.
6. For a race-free version use a tiny Lua script (since 2.6): `local c = redis.call("incr",KEYS[1]); if c == 1 then redis.call("expire",KEYS[1],1) end return c`.
7. Native integer storage: Redis stores the number in integer representation, so no string parse overhead; limited to 64-bit signed integers.
8. python: `r.incr('k')`, `r.incrby('k',5)`, `r.expire('k',ttl)`.