Transition rules manager to rule to client relationship

[bracyw]

Description

Currently the rules manager uses a client to rule structure that requires O(all_clients) iterations for each MQTT message:

clients_map: FxHashMap<ClientId, FxHashMap<Topic, Vec<Rule>>>,
rules_lookup: FxHashMap<Topic, FxHashSet<ClientId>>,

The current timing is not the worst as our client subscription size would most likely be at a max of 10 for the foreseeable future, however, logic-wise this is helpful and ultimately good to support early on.

Transition to:

pub struct RuleManager {
    rules: FxHashMap<RuleId, Rule>,
    topic_index: RwLock<FxHashMap<Topic, FxHashSet<RuleId>>>,
    subscriptions: RwLock<BiMultiMap<RuleId, ClientId>>,
}

BiMultiMap provides O(1) bidirectional lookups:

• Rule → Clients (for MQTT notifications)
• Client → Rules (for UI focused queries (what rules am I subbed too?)

Ultimately this will be transitioned to a database in the future, which should be seem less as the rules manager acts as a wrapper for whatever logic we decide it needs.

Acceptance Criteria

• functionality stays the same (test with Postman before changes)
• MQTT message processing uses O(1) topic lookup
• all operations stay within O(n) time
• concurrent MQTT processing enabled

Proposed Solution

1. Create BiMultiMap<L, R> struct (ask @bracyw (me) for more details)
2. Create new RuleManager structure with the new fields above
3. Convert old functions to logic for the new structure