Add memory limits and eviction policy

[AlexJuca]

Context

FKVS currently accepts writes until the process runs out of memory. Production deployments need predictable memory behavior: configured memory ceilings, metrics, and an eviction or rejection strategy.

Relevant code:

• src/core/hashtable.c
• src/server.h
• src/memory.c
• src/commands/server/server_command_handlers.c

Scope

• Add configurable maximum memory usage.
• Track memory used by keys, values, hash table entries, client buffers, TTL metadata, and persistence buffers where applicable.
• Decide policy when memory is exhausted: reject writes, evict keys, or support both modes.
• If eviction is implemented, define algorithm and metadata requirements (e.g. approximate LRU/LFU/random/TTL-first).
• Expose memory limit, current memory, rejected writes, and evictions through INFO/metrics.

Acceptance criteria

• A configured memory limit is enforced deterministically.
• Write commands fail gracefully or evict according to configuration when the limit is reached.
• Memory accounting is covered by tests.
• Behavior is documented in README/server config docs.