V1: Estimate standalone solar power budget

[mastertape]

Roadmap reference: docs/roadmap/dhl-paketkasten-retrofit-v1.md

Estimate whether the first standalone ESP-based DHL parcel box retrofit can be solar-supported.

Acceptance criteria:

• ESP idle/sleep and active current assumptions are listed.
• Keypad/RFID/fingerprint reader idle current is measured or estimated.
• Lock actuation peak current and duration are measured or estimated.
• Door sensor and tamper sensor current are listed.
• Battery chemistry and capacity options are compared.
• Charge controller / solar module assumptions are documented.
• Winter reserve target is proposed.
• Power design stays low-voltage only; no mains voltage on prototype boards.

Non-goal:
No final outdoor-certified power design is required in this issue.