Add missing CLI commands for TTL, transactions, indexes, and value search

[ElioNeto]

Problem

The CLI (src/cli/mod.rs) has 14 subcommands but is missing support for features recently added to the REST API and engine:

Feature	REST API	CLI?
TTL	GET/PATCH /keys/{key}/ttl	❌
Transactions	POST /txn + /commit /rollback	❌
Secondary Indexes	POST /keys/{key}/index/{field}, GET /keys/by-index	❌
Value Search	GET /keys/value-search	❌
Key Search Modes	GET /keys/search?mode=contains|regex	❌
Batch Delete	POST /keys/batch/delete	❌
Prefix Delete	DELETE /keys?prefix=	❌
Notes CRUD	Via API only	❌

Request

Add the following CLI subcommands:

TTL

• ttl set <key> <ttl_secs> — set or update TTL on existing key (calls PATCH /keys/{key}/ttl)
• ttl get <key> — get remaining TTL (calls GET /keys/{key}/ttl)
• set <key> <value> [--ttl <secs>] — extend existing set command with optional --ttl flag

Transactions

• txn begin — begin transaction, prints txn_id
• txn put <txn_id> <key> <value> — stage write
• txn delete <txn_id> <key> — stage delete
• txn commit <txn_id> — commit
• txn rollback <txn_id> — rollback

Indexes

• index create <field> — create secondary index on JSON field
• index query <field> <value> — query by index
• index list — list all indexes

Search

• search <query> [--mode prefix|contains|regex] [--limit N] — extend existing search with mode flag
• value-search <query> — search by value content

Batch/Prefix operations

• batch-delete <key1> [<key2> ...] — delete specific keys
• delete-prefix <prefix> — delete all keys matching prefix

Acceptance criteria

• Each new subcommand is tested via assert_cmd or similar CLI test framework
• Commands that call REST use reqwest (already a dependency) or call engine directly
• Help text (--help) is comprehensive and consistent
• Error messages are user-friendly
• Version string shown in CLI matches package version
• Use clap (already a dependency) for argument parsing