⚡ Optimize subscription assignment loop in receiver db

[iwismer]

💡 What:
Optimized replace_stream_subscriptions in services/receiver/src/db.rs by utilizing prepare_cached and direct low-level parameter binding (stmt.raw_bind_parameter(...) and stmt.raw_execute()) instead of allocating dynamic parameter slices (rusqlite::params![]) on every iteration of the subscription insertion loop.

🎯 Why:
When inserting multiple subscriptions, creating dynamic parameter vectors/slices (params![...]) on every row execution caused redundant parameter allocation and dynamic dispatch overhead. Switching to raw parameter binding and cached statements removes loop allocations while preserving exact functionality and parameter safety.

📊 Measured Improvement:

• Benchmark: bench_replace_stream_subscriptions (1,000 subscriptions, 20 iterations)
• Debug build: Reduced average runtime per batch from ~8.85 ms to ~6.07 ms (~31% speedup / 1.46x faster).
• Release build: Reduced average runtime per batch from ~1.89 ms to ~1.72 ms (~9% speedup / 1.10x faster).

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PR created automatically by Jules for task 12974985993301861592 started by @iwismer

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