feat: add dvhubonline feed-in tariff provider (#1193)

Fetches GET /api/prices?start&end&zone (15-min slots, EUR/MWh) and stores
the raw market price as feed_in_tariff_wh (EUR/Wh) — no import charges/VAT.

Slots beyond the day-ahead horizon are left to the consumer's forward-fill
(FeedInTariffImport behaviour).

5 unit tests + opt-in live smoke (EOS_DVHUB_ONLINE_LIVE=1, passing).

Signed-off-by: Christin <info@bikinibottom.capital>
Signed-off-by: Bobby Noelte <b0661n0e17e@gmail.com>

tests/testdata/docs/_generated/configexample.md

@@ -130,6 +130,10 @@
                "import_file_path": null,
                "import_json": null
            },
+           "dvhubonline": {
+               "base_url": "https://dvhub.online",
+               "zone": "DE-LU"
+           },
            "energycharts": {
                "bidding_zone": "DE-LU"
            }