refactor: consolidate ExecutionQueue + SlotService + BacklogService into CapacityManager

[vybe]

Summary

Three parallel queue/capacity abstractions exist in src/backend/services/:

• ExecutionQueue — Redis LIST, serial N=1, used by /chat
• SlotService — Redis ZSET, parallel N=max_parallel_tasks, used by /task
• BacklogService — SQLite FIFO, persistent overflow when slots full

These are one primitive with two knobs (max_concurrent, overflow_policy) expressed as three classes with overlapping TTL logic, overlapping drain paths, and inconsistent interfaces.

Scope

Merge into a single CapacityManager with:

CapacityManager(agent_name, max_concurrent=N, overflow_policy='reject' | 'queue_in_memory' | 'queue_persistent')

• /chat becomes CapacityManager(name, 1, 'queue_in_memory')
• /task becomes CapacityManager(name, max_parallel, 'queue_persistent')
• One TTL reasoner, one drain callback, one counter source

Depends on

• feat: Redis Streams event bus for reliable WebSocket delivery (RELIABILITY-003) #306 — push-based completion must land first. Much of the current TTL/drain machinery exists to compensate for blocking HTTP dispatch; the consolidation gets much smaller once feat: Redis Streams event bus for reliable WebSocket delivery (RELIABILITY-003) #306 moves state transitions to the event consumer.

Non-goals (keep v1 tight)

• Priority queueing (defer to v2)
• Cross-agent capacity pooling (covered by Horizontal Agent Scalability #24)
• Changing the public /chat vs /task semantics

Migration approach (additive-first)

1. Land CapacityManager class alongside the existing three classes.
2. Migrate callers one at a time behind a per-agent feature flag.
3. Keep old Queue/Slot/Backlog classes until zero callers remain.
4. Delete in a separate PR after a soak period.

No multi-PR intermediate states where both paths are partially wired.

Success criteria

• Grep for direct SlotService / ExecutionQueue / BacklogService instantiation returns zero hits outside CapacityManager and its tests.
• /chat and /task semantics unchanged observable-from-outside.
• Single drain path on capacity release.

Context

See docs/planning/ORCHESTRATION_RELIABILITY_2026-04.md — Tier 2.5 Simplification.

[vybe]

/claim