Flowtrack: private period tracker with cycle predictions

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

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+# Flowtrack
+
+A private, self-hosted period tracker. It logs period days with flow and
+symptoms, predicts the next six periods, and estimates the fertile window.
+It installs on a phone home screen as a web app.
+
+The app has no runtime dependencies beyond the Python standard library, and it
+stores everything in one SQLite file. Nginx and Authelia provide the access
+boundary.
+
+## Predictions
+
+- Periods are runs of logged period days. Spotting doesn't count, and one
+  missing day inside a run doesn't split it.
+- The cycle length is a weighted average of up to the last 12 cycles, with
+  recent cycles counting more. Cycles shorter than 15 days or longer than 60
+  are left out because they usually mean a log was missed.
+- Each prediction is a range of ± the cycle's standard deviation (at least 2
+  days, or 4 with fewer than three cycles). The range widens with √n for later
+  cycles.
+- Ovulation is estimated 14 days before the next period. The fertile window
+  runs from 5 days before ovulation to 1 day after. These are estimates, not
+  contraception.
+- Once the expected day passes, the next period is expected from tomorrow,
+  and later predictions move with it.
+
+The logic lives in `static/cycle.js` and is tested in `tests/test_cycle.js`.
+
+## Access
+
+Nginx must overwrite `Remote-User` with the Authelia username. Users listed
+with `--editor` can log and export. Users listed with `--viewer` see period
+days and predictions only. They also see flow and symptoms if the owner turns
+on sharing in Settings. Everyone else gets 403.
+
+## Run locally
+
+```sh
+python3 app.py --database /tmp/flowtrack.sqlite3 --editor me --dev-user me
+```
+
+Then open `http://127.0.0.1:8093`.
+
+## NixOS module
+
+```nix
+services.flowtrack = {
+  enable = true;
+  editors = [ "kayla" ];
+  viewers = [ "fred" ];
+  backup.directory = "/mnt/disk2/backups/flowtrack";  # nightly, keeps 30
+};
+```
+
+`python3 app.py --database DB --backup-to DIR --keep N` takes a consistent
+snapshot by hand.