Support new MedControl API format, dynamic ring groups, and multi-recipient extensions

[lramos15]

Summary\n\nAdapts DiALERT to the upcoming MedControl scheduling API changes and generalizes the PBX ring group configuration.\n\n## Changes\n\n### 1. Support both old and new MedControl API shapes\n\nThe GET /api/shifts/current endpoint is moving from a flat recipients[] structure to a groups[] structure where each group can contain multiple recipients. getCurrentSchedule now detects which format was returned (body.groups vs body.recipients) and normalizes both into a consistent internal groups representation.\n\n### 2. Multi-recipient ring groups\n\nEach ring group can now simultaneously call multiple people. The extensionList is built by joining all recipient numbers in a group (e.g. 5551234567#-5559876543#), and the existing ringall strategy handles simultaneous ringing.\n\n### 3. Dynamic ring group naming via RING_GROUP_PREFIX\n\nReplaces the hardcoded RG1/RG2/RG3 env vars with a single RING_GROUP_PREFIX. Ring group numbers are computed as <prefix><priority> (e.g. prefix 100 → groups 1001, 1002, 1003, …). This also removes the previous 3-group cap — the system now supports any number of groups.\n\n### 4. Generalized chaining logic\n\nThe ring group overflow chaining (postAnswer) is no longer a set of hardcoded switch/if branches for 1–3 recipients. It now works for any count: each group chains to the next, and the last group terminates with app-blackhole,busy,1.\n\n## Env changes\n\ndiff\n- RG1=1001\n- RG2=1002\n- RG3=1003\n+ RING_GROUP_PREFIX=100\n