add power system: IP5389 board + 18650 array decision

- IP5389 power bank board selected (preliminary)
- All-in-one: charging, BMS, USB-C PD output
- Rejected K501U battery (integrated BMS not reusable)
- Documented alternatives considered (TP5100+2S BMS+buck, UPS HAT)
- Updated parts list with power system BOM
- Cost estimate now 06-230 (was 70-190)
- Added power-system.md, updated design-decisions.md

docs/parts-list.md

@@ -32,8 +32,10 @@
 
 | Item | Specification | Source | Est. Cost | Status |
 |------|--------------|--------|-----------|--------|
-| Power supply | USB-C PD or 12V 2A | Amazon | $8-12 | To purchase |
-| (Future) Battery | 10,000-20,000 mAh PD | TBD | $20-40 | Future |
+| IP5389 power board | USB-C PD, 3S, bare PCB | AliExpress | $8-15 | Preliminary selection |
+| 18650 cells | 3S or 3S2P, quality brand | User's pick | $12-24 | To purchase |
+| 5A blade fuse + holder | Automotive | Amazon | $2 | To purchase |
+| USB-C wall charger | 45W+ PD, for charging | Amazon | $10-15 | To purchase |
 
 ### Networking
 
@@ -62,24 +64,24 @@
 | MicroSD | $12 |
 | USB hub | $20 |
 | USB enclosure | $12 |
-| Power supply | $10 |
+| Power system (board+cells+fuse+charger) | $32-56 |
 | WiFi dongle | $12 |
-| **Subtotal** | **$194** |
+| **Subtotal** | **$206-230** |
 
 ### With Optional Items
 
 | Category | Cost |
 |----------|------|
-| Base build | $194 |
+| Base build | $206-230 |
 | Heatsink/cooling | $10 |
 | Panel-mount cables | $15 |
-| **Total** | **$219** |
+| **Total** | **$231-255** |
 
 ### Future Additions
 
 | Category | Cost |
 |----------|------|
-| Battery pack (portable) | $20-40 |
+| Battery pack (portable) | Integrated in base build |
 | Case materials | TBD |
 | Custom PCB | TBD |
 
@@ -96,7 +98,9 @@
 - MicroSD card
 - Powered USB hub
 - USB enclosure or flash drive
-- Power supply
+- IP5389 power board + 18650 cells
+- 5A fuse + holder
+- USB-C PD wall charger (45W+)
 - WiFi dongle
 
 ---